diff --git a/accessible/tests/browser/head.js b/accessible/tests/browser/head.js
index 0524b4789c58..6ad237096351 100644
--- a/accessible/tests/browser/head.js
+++ b/accessible/tests/browser/head.js
@@ -130,8 +130,14 @@ function waitForEvent(eventType, expectedId) {
     let eventObserver = {
       observe(subject) {
         let event = subject.QueryInterface(Ci.nsIAccessibleEvent);
+        let id;
+        try {
+          id = event.accessible.id;
+        } catch (e) {
+          // This can throw NS_ERROR_FAILURE.
+        }
         if (event.eventType === eventType &&
-            event.accessible.id === expectedId) {
+            id === expectedId) {
           Services.obs.removeObserver(this, "accessible-event");
           resolve(event);
         }
diff --git a/browser/base/content/test/alerts/browser_notification_do_not_disturb.js b/browser/base/content/test/alerts/browser_notification_do_not_disturb.js
index d66e12304bb6..7b23e96262e5 100644
--- a/browser/base/content/test/alerts/browser_notification_do_not_disturb.js
+++ b/browser/base/content/test/alerts/browser_notification_do_not_disturb.js
@@ -35,12 +35,11 @@ function test() {
   addNotificationPermission(notificationURL).then(function openTab() {
     tab = BrowserTestUtils.addTab(gBrowser, notificationURL);
     gBrowser.selectedTab = tab;
-    tab.linkedBrowser.addEventListener("load", onLoad, true);
+    BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => onLoad());
   });
 }
 
 function onLoad() {
-  tab.linkedBrowser.removeEventListener("load", onLoad, true);
   openNotification(tab.linkedBrowser, "showNotification2").then(onAlertShowing);
 }
 
diff --git a/browser/base/content/test/alerts/browser_notification_remove_permission.js b/browser/base/content/test/alerts/browser_notification_remove_permission.js
index a6fdd5042edb..50e128404c16 100644
--- a/browser/base/content/test/alerts/browser_notification_remove_permission.js
+++ b/browser/base/content/test/alerts/browser_notification_remove_permission.js
@@ -16,12 +16,11 @@ function test() {
   addNotificationPermission(notificationURL).then(function openTab() {
     tab = BrowserTestUtils.addTab(gBrowser, notificationURL);
     gBrowser.selectedTab = tab;
-    tab.linkedBrowser.addEventListener("load", onLoad, true);
+    BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => onLoad());
   });
 }
 
 function onLoad() {
-  tab.linkedBrowser.removeEventListener("load", onLoad, true);
   openNotification(tab.linkedBrowser, "showNotification2").then(onAlertShowing);
 }
 
diff --git a/browser/base/content/test/general/browser_blockHPKP.js b/browser/base/content/test/general/browser_blockHPKP.js
index 26c97cd56a24..a545fe678c7f 100644
--- a/browser/base/content/test/general/browser_blockHPKP.js
+++ b/browser/base/content/test/general/browser_blockHPKP.js
@@ -49,9 +49,7 @@ function test() {
 function loadPinningPage() {
 
   BrowserTestUtils.loadURI(gBrowser.selectedBrowser, "https://" + kPinningDomain + kURLPath + "valid").then(function() {
-    gBrowser.selectedBrowser.addEventListener("load",
-                                               successfulPinningPageListener,
-                                               true);
+      BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => successfulPinningPageListener.handleEvent());
   });
 }
 
@@ -59,7 +57,6 @@ function loadPinningPage() {
 // fail to validate
 var successfulPinningPageListener = {
   handleEvent() {
-    gBrowser.selectedBrowser.removeEventListener("load", this, true);
     BrowserTestUtils.loadURI(gBrowser.selectedBrowser, "https://" + kBadPinningDomain).then(function() {
       return BrowserTestUtils.waitForErrorPage(gBrowser.selectedBrowser);
     }).then(errorPageLoaded);
diff --git a/browser/base/content/test/general/browser_bug406216.js b/browser/base/content/test/general/browser_bug406216.js
index 96a69c4f6325..d010950df29b 100644
--- a/browser/base/content/test/general/browser_bug406216.js
+++ b/browser/base/content/test/general/browser_bug406216.js
@@ -24,10 +24,10 @@ function addTab(aURI, aIndex) {
   if (aIndex == 0)
     gBrowser.removeTab(gBrowser.tabs[0], {skipPermitUnload: true});
 
-  tab.linkedBrowser.addEventListener("load", function(event) {
+  BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => {
     if (++count == URIS.length)
       executeSoon(doTabsTest);
-  }, {capture: true, once: true});
+  });
 }
 
 function doTabsTest() {
diff --git a/browser/base/content/test/general/browser_bug432599.js b/browser/base/content/test/general/browser_bug432599.js
index 8853c958070c..ee92798a6ffa 100644
--- a/browser/base/content/test/general/browser_bug432599.js
+++ b/browser/base/content/test/general/browser_bug432599.js
@@ -45,11 +45,11 @@ function test() {
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => {
     waitForStarChange(false, initTest);
-  }, {capture: true, once: true});
+  });
 
-  content.location = testURL;
+  gBrowser.loadURI(testURL);
 }
 
 function initTest() {
diff --git a/browser/base/content/test/general/browser_bug479408.js b/browser/base/content/test/general/browser_bug479408.js
index 77c607b6499c..1c81f1ecdadc 100644
--- a/browser/base/content/test/general/browser_bug479408.js
+++ b/browser/base/content/test/general/browser_bug479408.js
@@ -3,7 +3,7 @@ function test() {
   let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser,
     "http://mochi.test:8888/browser/browser/base/content/test/general/browser_bug479408_sample.html");
 
-  gBrowser.addEventListener("DOMLinkAdded", function(aEvent) {
+  BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "DOMLinkAdded", true).then(() => {
     executeSoon(function() {
       ok(!tab.linkedBrowser.engines,
          "the subframe's search engine wasn't detected");
@@ -11,5 +11,5 @@ function test() {
       gBrowser.removeTab(tab);
       finish();
     });
-  }, {capture: true, once: true});
+  });
 }
diff --git a/browser/base/content/test/general/browser_bug567306.js b/browser/base/content/test/general/browser_bug567306.js
index 54a1305638cc..16e6c26ba357 100644
--- a/browser/base/content/test/general/browser_bug567306.js
+++ b/browser/base/content/test/general/browser_bug567306.js
@@ -11,14 +11,12 @@ add_task(async function() {
 
   let selectedBrowser = newwindow.gBrowser.selectedBrowser;
   await new Promise((resolve, reject) => {
-    selectedBrowser.addEventListener("pageshow", function pageshowListener() {
-      if (selectedBrowser.currentURI.spec == "about:blank")
-        return;
-
-      selectedBrowser.removeEventListener("pageshow", pageshowListener, true);
-      ok(true, "pageshow listener called: " + newwindow.content.location);
+    BrowserTestUtils.waitForContentEvent(selectedBrowser, "pageshow", true, (event) => {
+      return content.location.href != "about:blank";
+    }).then(function pageshowListener() {
+      ok(true, "pageshow listener called: " + newwindow.gBrowser.currentURI.spec);
       resolve();
-    }, true);
+    });
     selectedBrowser.loadURI("data:text/html,<h1 id='h1'>Select Me</h1>");
   });
 
diff --git a/browser/base/content/test/general/browser_bug581253.js b/browser/base/content/test/general/browser_bug581253.js
index 3708727072f7..8cfbcf7c990b 100644
--- a/browser/base/content/test/general/browser_bug581253.js
+++ b/browser/base/content/test/general/browser_bug581253.js
@@ -24,8 +24,7 @@ add_task(async function test_remove_bookmark_with_tag_via_edit_bookmark() {
 
   Assert.ok(await PlacesUtils.bookmarks.fetch({url: testURL}), "the test url is bookmarked");
 
-  // eslint-disable-next-line mozilla/no-cpows-in-tests
-  content.location = testURL;
+  gBrowser.loadURI(testURL);
 
   await BrowserTestUtils.waitForCondition(
     () => BookmarkingUI.status == BookmarkingUI.STATUS_STARRED,
diff --git a/browser/base/content/test/general/browser_bug767836_perwindowpb.js b/browser/base/content/test/general/browser_bug767836_perwindowpb.js
index 8307277ac2da..923de42f9450 100644
--- a/browser/base/content/test/general/browser_bug767836_perwindowpb.js
+++ b/browser/base/content/test/general/browser_bug767836_perwindowpb.js
@@ -76,12 +76,13 @@ function openNewTab(aWindow, aCallback) {
 
   let browser = aWindow.gBrowser.selectedBrowser;
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  if (browser.contentDocument.readyState === "complete") {
+  let doc = browser.contentDocumentAsCPOW;
+  if (doc && doc.readyState === "complete") {
     executeSoon(aCallback);
     return;
   }
 
-  browser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(browser).then(() => {
     executeSoon(aCallback);
-  }, {capture: true, once: true});
+  });
 }
diff --git a/browser/base/content/test/general/browser_bug817947.js b/browser/base/content/test/general/browser_bug817947.js
index 24d118f0cd97..ce28c747b9f6 100644
--- a/browser/base/content/test/general/browser_bug817947.js
+++ b/browser/base/content/test/general/browser_bug817947.js
@@ -47,8 +47,8 @@ function preparePendingTab(aCallback) {
   });
 }
 
-function whenLoaded(aElement, aCallback) {
-  aElement.addEventListener("load", function() {
+function whenLoaded(aBrowser, aCallback) {
+  BrowserTestUtils.browserLoaded(aBrowser).then(() => {
     executeSoon(aCallback);
-  }, {capture: true, once: true});
+  });
 }
diff --git a/browser/base/content/test/general/browser_favicon_change_not_in_document.js b/browser/base/content/test/general/browser_favicon_change_not_in_document.js
index 55c0288fadd4..3ad14277bcd7 100644
--- a/browser/base/content/test/general/browser_favicon_change_not_in_document.js
+++ b/browser/base/content/test/general/browser_favicon_change_not_in_document.js
@@ -20,8 +20,8 @@ add_task(async function() {
   let domLinkChangedFired = 0;
   const linkAddedHandler = event => domLinkAddedFired++;
   const linkChangedhandler = event => domLinkChangedFired++;
-  gBrowser.addEventListener("DOMLinkAdded", linkAddedHandler);
-  gBrowser.addEventListener("DOMLinkChanged", linkChangedhandler);
+  BrowserTestUtils.addContentEventListener(gBrowser.selectedBrowser, "DOMLinkAdded", linkAddedHandler);
+  BrowserTestUtils.addContentEventListener(gBrowser.selectedBrowser, "DOMLinkChanged", linkChangedhandler);
   extraTab.linkedBrowser.loadURI(TEST_URL);
   let expectedFavicon = "http://example.org/yet-another-icon";
   await promiseTabLoaded(extraTab);
@@ -39,7 +39,5 @@ add_task(async function() {
   is(domLinkAddedFired, 2, "Should fire the correct number of DOMLinkAdded event.");
   is(domLinkChangedFired, 0, "Should not fire any DOMLinkChanged event.");
 
-  gBrowser.removeEventListener("DOMLinkAdded", linkAddedHandler);
-  gBrowser.removeEventListener("DOMLinkChanged", linkChangedhandler);
   gBrowser.removeTab(extraTab);
 });
diff --git a/browser/base/content/test/general/browser_gZipOfflineChild.js b/browser/base/content/test/general/browser_gZipOfflineChild.js
index 5dcb1cf45f5e..5312e8a9b5a8 100644
--- a/browser/base/content/test/general/browser_gZipOfflineChild.js
+++ b/browser/base/content/test/general/browser_gZipOfflineChild.js
@@ -13,16 +13,20 @@ registerCleanupFunction(function() {
   Services.prefs.clearUserPref("offline-apps.allow_by_default");
 });
 
-var cacheCount = 0;
-var intervalID = 0;
-
 //
 // Handle "message" events which are posted from the iframe upon
 // offline cache events.
 //
-function handleMessageEvents(event) {
-  cacheCount++;
-  switch (cacheCount) {
+function contentTask() {
+  let resolve;
+  let promise = new Promise(r => { resolve = r; });
+
+  var cacheCount = 0;
+  var intervalID = 0;
+
+  function handleMessageEvents(event) {
+    cacheCount++;
+    switch (cacheCount) {
     case 1:
       // This is the initial caching off offline data.
       is(event.data, "oncache", "Child was successfully cached.");
@@ -54,12 +58,16 @@ function handleMessageEvents(event) {
     case 2:
       is(event.data, "onupdate", "Child was successfully updated.");
       clearInterval(intervalID);
-      finish();
+      resolve();
       break;
     default:
       // how'd we get here?
       ok(false, "cacheCount not 1 or 2");
+    }
   }
+
+  content.addEventListener("message", handleMessageEvents);
+  return promise;
 }
 
 function test() {
@@ -72,9 +80,6 @@ function test() {
   registerCleanupFunction(() => gBrowser.removeCurrentTab());
 
   BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => {
-    // eslint-disable-next-line mozilla/no-cpows-in-tests
-    let window = gBrowser.selectedBrowser.contentWindow;
-
-    window.addEventListener("message", handleMessageEvents);
+    ContentTask.spawn(gBrowser.selectedBrowser, null, contentTask).then(finish);
   });
 }
diff --git a/browser/base/content/test/general/browser_keywordBookmarklets.js b/browser/base/content/test/general/browser_keywordBookmarklets.js
index 4594c9d45706..c995c38eeb3e 100644
--- a/browser/base/content/test/general/browser_keywordBookmarklets.js
+++ b/browser/base/content/test/general/browser_keywordBookmarklets.js
@@ -45,9 +45,5 @@ add_task(async function test_keyword_bookmarklet() {
 });
 
 function promisePageShow() {
-  return new Promise(resolve => {
-    gBrowser.selectedBrowser.addEventListener("pageshow", function() {
-      resolve();
-    }, {once: true});
-  });
+  return BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "pageshow");
 }
diff --git a/browser/base/content/test/general/browser_keywordSearch.js b/browser/base/content/test/general/browser_keywordSearch.js
index 65d49374edef..e4a2c0a0ebf2 100644
--- a/browser/base/content/test/general/browser_keywordSearch.js
+++ b/browser/base/content/test/general/browser_keywordSearch.js
@@ -36,6 +36,45 @@ function test() {
 
   let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
 
+  // We use a web progress listener in the content process to cancel
+  // the request. For everything else, we can do the work in the
+  // parent, since it's easier.
+  ContentTask.spawn(gBrowser.selectedBrowser, null, function* gen() {
+    const Ci = Components.interfaces;
+
+    let listener = {
+      onStateChange: function onLocationChange(webProgress, req, flags, status) {
+        let docStart = Ci.nsIWebProgressListener.STATE_IS_DOCUMENT |
+                       Ci.nsIWebProgressListener.STATE_START;
+        if (!(flags & docStart)) {
+          return;
+        }
+
+        req.cancel(Components.results.NS_ERROR_FAILURE);
+      },
+
+      QueryInterface: function QueryInterface(aIID) {
+        if (aIID.equals(Ci.nsIWebProgressListener) ||
+            aIID.equals(Ci.nsIWebProgressListener2) ||
+            aIID.equals(Ci.nsISupportsWeakReference) ||
+            aIID.equals(Ci.nsISupports)) {
+          return this;
+        }
+
+        throw Components.results.NS_ERROR_NO_INTERFACE;
+      }
+    };
+
+    let webProgress = docShell.QueryInterface(Ci.nsIInterfaceRequestor)
+        .getInterface(Ci.nsIWebProgress);
+    webProgress.addProgressListener(listener,
+                                    Ci.nsIWebProgress.NOTIFY_STATE_DOCUMENT);
+
+    addEventListener("unload", () => {
+      webProgress.removeProgressListener(listener);
+    });
+  });
+
   let listener = {
     onStateChange: function onLocationChange(webProgress, req, flags, status) {
       // Only care about document starts
@@ -50,11 +89,10 @@ function test() {
       is(req.originalURI.spec, gCurrTest.searchURL, "search URL was loaded");
       info("Actual URI: " + req.URI.spec);
 
-      req.cancel(Components.results.NS_ERROR_FAILURE);
-
       executeSoon(nextTest);
     }
   };
+
   gBrowser.addProgressListener(listener);
 
   registerCleanupFunction(function() {
diff --git a/browser/base/content/test/general/browser_keywordSearch_postData.js b/browser/base/content/test/general/browser_keywordSearch_postData.js
index fdf3ff54dbac..13c453ee719e 100644
--- a/browser/base/content/test/general/browser_keywordSearch_postData.js
+++ b/browser/base/content/test/general/browser_keywordSearch_postData.js
@@ -68,7 +68,7 @@ function doTest() {
 
   waitForLoad(function() {
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    let loadedText = gBrowser.contentDocument.body.textContent;
+    let loadedText = gBrowser.contentDocumentAsCPOW.body.textContent;
     ok(loadedText, "search page loaded");
     let needle = "searchterms=" + gCurrTest.expectText;
     is(loadedText, needle, "The query POST data should be returned in the response");
@@ -84,12 +84,11 @@ function doTest() {
 
 function waitForLoad(cb) {
   let browser = gBrowser.selectedBrowser;
-  browser.addEventListener("load", function listener() {
-    if (browser.currentURI.spec == "about:blank")
-      return;
+  function wantLoad(url) {
+    return url != "about:blank";
+  }
+  BrowserTestUtils.browserLoaded(browser, false, wantLoad).then(() => {
     info("Page loaded: " + browser.currentURI.spec);
-    browser.removeEventListener("load", listener, true);
-
     cb();
   }, true);
 }
diff --git a/browser/base/content/test/general/browser_star_hsts.js b/browser/base/content/test/general/browser_star_hsts.js
index 90ce54a6df8f..42999ef777ea 100644
--- a/browser/base/content/test/general/browser_star_hsts.js
+++ b/browser/base/content/test/general/browser_star_hsts.js
@@ -66,19 +66,8 @@ function promiseStarState(aValue) {
 function promiseTabLoadEvent(aTab, aURL, aFinalURL) {
   if (!aFinalURL)
     aFinalURL = aURL;
-  return new Promise(resolve => {
-    info("Wait for load tab event");
-    aTab.linkedBrowser.addEventListener("load", function load(event) {
-      if (event.originalTarget != aTab.linkedBrowser.contentDocument ||
-          event.target.location.href == "about:blank" ||
-          event.target.location.href != aFinalURL) {
-        info("skipping spurious load event");
-        return;
-      }
-      aTab.linkedBrowser.removeEventListener("load", load, true);
-      info("Tab load event received");
-      resolve();
-    }, true, true);
-    aTab.linkedBrowser.loadURI(aURL);
-  });
+
+  info("Wait for load tab event");
+  aTab.linkedBrowser.loadURI(aURL);
+  return BrowserTestUtils.browserLoaded(aTab.linkedBrowser, false, aFinalURL);
 }
diff --git a/browser/base/content/test/general/browser_tab_dragdrop2.js b/browser/base/content/test/general/browser_tab_dragdrop2.js
index e6fb8dcdeef0..5ddc76d62ce9 100644
--- a/browser/base/content/test/general/browser_tab_dragdrop2.js
+++ b/browser/base/content/test/general/browser_tab_dragdrop2.js
@@ -39,7 +39,7 @@ add_task(async function() {
 
   // Run tests once again.
   let promise = promiseTestsDone(win2);
-  win2.content.test_panels();
+  win2.gBrowser.contentWindowAsCPOW.test_panels();
   await promise;
   ok(true, "tests succeeded a second time");
 
diff --git a/browser/base/content/test/general/browser_tabfocus.js b/browser/base/content/test/general/browser_tabfocus.js
index d5b942bcbb26..6b48c543e2aa 100644
--- a/browser/base/content/test/general/browser_tabfocus.js
+++ b/browser/base/content/test/general/browser_tabfocus.js
@@ -329,9 +329,7 @@ add_task(async function() {
   gURLBar.focus();
 
   await new Promise((resolve, reject) => {
-    window.addEventListener("pageshow", function(event) {
-      resolve();
-    }, {capture: true, once: true});
+    BrowserTestUtils.waitForContentEvent(window.gBrowser.selectedBrowser, "pageshow", true).then(() => resolve());
     document.getElementById("Browser:Back").doCommand();
   });
 
diff --git a/browser/base/content/test/general/head.js b/browser/base/content/test/general/head.js
index ab957f6d6121..32fe182fb091 100644
--- a/browser/base/content/test/general/head.js
+++ b/browser/base/content/test/general/head.js
@@ -314,7 +314,8 @@ function whenNewTabLoaded(aWindow, aCallback) {
   aWindow.BrowserOpenTab();
 
   let browser = aWindow.gBrowser.selectedBrowser;
-  if (browser.contentDocument.readyState === "complete") {
+  let doc = browser.contentDocumentAsCPOW;
+  if (doc && doc.readyState === "complete") {
     aCallback();
     return;
   }
@@ -433,11 +434,11 @@ var FullZoomHelper = {
       let didPs = false;
       let didZoom = false;
 
-      gBrowser.addEventListener("pageshow", function(event) {
+      BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "pageshow", true).then(() => {
         didPs = true;
         if (didZoom)
           resolve();
-      }, {capture: true, once: true});
+      });
 
       if (direction == this.BACK)
         gBrowser.goBack();
diff --git a/browser/base/content/test/newtab/head.js b/browser/base/content/test/newtab/head.js
index 359bdc4402c7..a5f1628c850b 100644
--- a/browser/base/content/test/newtab/head.js
+++ b/browser/base/content/test/newtab/head.js
@@ -40,50 +40,59 @@ requiredSize.innerWidth =
   (3 * (290 + 20)) + // 3 cols * (tile width + side margins)
   100; // breathing room
 
-var oldSize = {};
-Object.keys(requiredSize).forEach(prop => {
-  info([prop, gBrowser.contentWindow[prop], requiredSize[prop]]);
-  if (gBrowser.contentWindow[prop] < requiredSize[prop]) {
-    oldSize[prop] = gBrowser.contentWindow[prop];
-    info("Changing browser " + prop + " from " + oldSize[prop] + " to " +
-         requiredSize[prop]);
-    gBrowser.contentWindow[prop] = requiredSize[prop];
-  }
-});
-
-var screenHeight = {};
-var screenWidth = {};
-Cc["@mozilla.org/gfx/screenmanager;1"].
-  getService(Ci.nsIScreenManager).
-  primaryScreen.
-  GetAvailRectDisplayPix({}, {}, screenWidth, screenHeight);
-screenHeight = screenHeight.value;
-screenWidth = screenWidth.value;
-
-if (screenHeight < gBrowser.contentWindow.outerHeight) {
-  info("Warning: Browser outer height is now " +
-       gBrowser.contentWindow.outerHeight + ", which is larger than the " +
-       "available screen height, " + screenHeight +
-       ". That may cause problems.");
-}
-
-if (screenWidth < gBrowser.contentWindow.outerWidth) {
-  info("Warning: Browser outer width is now " +
-       gBrowser.contentWindow.outerWidth + ", which is larger than the " +
-       "available screen width, " + screenWidth +
-       ". That may cause problems.");
-}
-
-registerCleanupFunction(function() {
-  while (gWindow.gBrowser.tabs.length > 1)
-    gWindow.gBrowser.removeTab(gWindow.gBrowser.tabs[1]);
-
-  Object.keys(oldSize).forEach(prop => {
-    if (oldSize[prop]) {
-      gBrowser.contentWindow[prop] = oldSize[prop];
-    }
+add_task(async function setupWindowSize() {
+  let [oldSize, curWidth, curHeight] = await ContentTask.spawn(gBrowser.selectedBrowser, requiredSize, (requiredSizeArg) => {
+    var oldSizeVar = {};
+    Object.keys(requiredSizeArg).forEach(prop => {
+      info([prop, content[prop], requiredSizeArg[prop]]);
+      if (content[prop] < requiredSizeArg[prop]) {
+        oldSizeVar[prop] = content[prop];
+        info("Changing browser " + prop + " from " + oldSizeVar[prop] + " to " +
+             requiredSizeArg[prop]);
+        content[prop] = requiredSizeArg[prop];
+      }
+    });
+    return [oldSizeVar, content.outerWidth, content.outerHeight];
   });
 
+  var screenHeight = {};
+  var screenWidth = {};
+  Cc["@mozilla.org/gfx/screenmanager;1"].
+    getService(Ci.nsIScreenManager).
+    primaryScreen.
+    GetAvailRectDisplayPix({}, {}, screenWidth, screenHeight);
+  screenHeight = screenHeight.value;
+  screenWidth = screenWidth.value;
+
+  if (screenHeight < curHeight) {
+    info("Warning: Browser outer height is now " +
+         curHeight + ", which is larger than the " +
+         "available screen height, " + screenHeight +
+         ". That may cause problems.");
+  }
+
+  if (screenWidth < curWidth) {
+    info("Warning: Browser outer width is now " +
+         curWidth + ", which is larger than the " +
+         "available screen width, " + screenWidth +
+         ". That may cause problems.");
+  }
+
+  registerCleanupFunction(function() {
+    while (gWindow.gBrowser.tabs.length > 1)
+      gWindow.gBrowser.removeTab(gWindow.gBrowser.tabs[1]);
+
+    ContentTask.spawn(gBrowser.selectedBrowser, oldSize, (oldSizeArg) => {
+      Object.keys(oldSizeArg).forEach(prop => {
+        if (oldSizeArg[prop]) {
+          content[prop] = oldSizeArg[prop];
+        }
+      });
+    });
+  });
+});
+
+registerCleanupFunction(function() {
   Services.prefs.clearUserPref(PREF_NEWTAB_ENABLED);
   Services.prefs.clearUserPref(PREF_NEWTAB_ACTIVITY_STREAM);
   Services.prefs.setCharPref(PREF_NEWTAB_DIRECTORYSOURCE, gOrigDirectorySource);
diff --git a/browser/base/content/test/pageinfo/browser_pageInfo.js b/browser/base/content/test/pageinfo/browser_pageInfo.js
index 765223723fa6..a2ebd43d22b8 100644
--- a/browser/base/content/test/pageinfo/browser_pageInfo.js
+++ b/browser/base/content/test/pageinfo/browser_pageInfo.js
@@ -4,12 +4,11 @@ function test() {
   var pageInfo;
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => {
     Services.obs.addObserver(observer, "page-info-dialog-loaded");
     pageInfo = BrowserPageInfo();
-  }, {capture: true, once: true});
-  content.location =
-    "https://example.com/browser/browser/base/content/test/pageinfo/feed_tab.html";
+  });
+  gBrowser.selectedBrowser.loadURI("https://example.com/browser/browser/base/content/test/pageinfo/feed_tab.html");
 
   function observer(win, topic, data) {
     Services.obs.removeObserver(observer, "page-info-dialog-loaded");
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js b/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js
index 7da1799e72cf..99f3f256d4e0 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_firstPartyIsolation.js
@@ -72,8 +72,8 @@ async function test() {
   });
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  content.location = "https://example.com/browser/browser/base/content/test/pageinfo/image.html";
-  await waitForEvent(gBrowser.selectedBrowser, "load");
+  gBrowser.selectedBrowser.loadURI("https://example.com/browser/browser/base/content/test/pageinfo/image.html");
+  await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
 
   let spec = gBrowser.selectedBrowser.currentURI.spec;
 
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js b/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js
index d9ad8f68e920..6e1ceffa35ea 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_image_info.js
@@ -13,9 +13,9 @@ function test() {
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
 
-  gBrowser.selectedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function() {
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    var doc = gBrowser.contentDocument;
+    var doc = gBrowser.contentDocumentAsCPOW;
     var testImg = doc.getElementById("test-image");
     var pageInfo = BrowserPageInfo(gBrowser.selectedBrowser.currentURI.spec,
                                    "mediaTab", getImageInfo(testImg));
@@ -35,14 +35,14 @@ function test() {
         });
       });
     }, {capture: true, once: true});
-  }, {capture: true, once: true});
+  });
 
-  content.location =
+  gBrowser.loadURI(
     "data:text/html," +
     "<style type='text/css'>%23test-image,%23not-test-image {background-image: url('about:logo?c');}</style>" +
     "<img src='about:logo?b' height=300 width=350 alt=2 id='not-test-image'>" +
     "<img src='about:logo?b' height=300 width=350 alt=2>" +
     "<img src='about:logo?a' height=200 width=250>" +
     "<img src='about:logo?b' height=200 width=250 alt=1>" +
-    "<img src='about:logo?b' height=100 width=150 alt=2 id='test-image'>";
+    "<img src='about:logo?b' height=100 width=150 alt=2 id='test-image'>");
 }
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_images.js b/browser/base/content/test/pageinfo/browser_pageinfo_images.js
index 053c3f85e9ff..78ce46265ee9 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_images.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_images.js
@@ -5,7 +5,7 @@ function test() {
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
 
-  gBrowser.selectedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => {
     var pageInfo = BrowserPageInfo(gBrowser.selectedBrowser.currentURI.spec,
                                    "mediaTab");
 
@@ -26,9 +26,9 @@ function test() {
         });
       });
     }, {capture: true, once: true});
-  }, {capture: true, once: true});
+  });
 
-  content.location =
+  gBrowser.selectedBrowser.loadURI(
     "data:text/html," +
     "<html>" +
     "  <head>" +
@@ -44,5 +44,5 @@ function test() {
     "    <a href='' style='cursor: url(about:logo?d),default;'>test link</a>" + // Cursor
     "    <object type='image/svg+xml' width=20 height=20 data='file:///dummy_object.svg'></object>" + // Object
     "  </body>" +
-    "</html>";
+    "</html>");
 }
diff --git a/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js b/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js
index 2552a873d924..a2129459d141 100644
--- a/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js
+++ b/browser/base/content/test/pageinfo/browser_pageinfo_svg_image.js
@@ -3,7 +3,7 @@ function test() {
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
 
-  gBrowser.selectedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => {
     var pageInfo = BrowserPageInfo(gBrowser.selectedBrowser.currentURI.spec,
                                    "mediaTab");
 
@@ -30,8 +30,8 @@ function test() {
         });
       });
     }, {capture: true, once: true});
-  }, {capture: true, once: true});
+  });
 
-  content.location =
-    "https://example.com/browser/browser/base/content/test/pageinfo/svg_image.html";
+  gBrowser.selectedBrowser.loadURI(
+    "https://example.com/browser/browser/base/content/test/pageinfo/svg_image.html");
 }
diff --git a/browser/base/content/test/plugins/browser_CTP_context_menu.js b/browser/base/content/test/plugins/browser_CTP_context_menu.js
index c928ff560dda..c510a13a07a9 100644
--- a/browser/base/content/test/plugins/browser_CTP_context_menu.js
+++ b/browser/base/content/test/plugins/browser_CTP_context_menu.js
@@ -21,7 +21,7 @@ add_task(async function() {
 
   Services.prefs.setBoolPref("plugins.click_to_play", true);
   setTestPluginEnabledState(Ci.nsIPluginTag.STATE_CLICKTOPLAY, "Test Plug-in");
-  let bindingPromise = waitForEvent(gBrowser.selectedBrowser, "PluginBindingAttached", null, true, true);
+  let bindingPromise = BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "PluginBindingAttached", true, null, true);
   await promiseTabLoadEvent(gBrowser.selectedTab, gTestRoot + "plugin_test.html");
   await promiseUpdatePluginBindings(gBrowser.selectedBrowser);
   await bindingPromise;
diff --git a/browser/base/content/test/plugins/browser_CTP_drag_drop.js b/browser/base/content/test/plugins/browser_CTP_drag_drop.js
index 61ddfcd0618e..27d188b40988 100644
--- a/browser/base/content/test/plugins/browser_CTP_drag_drop.js
+++ b/browser/base/content/test/plugins/browser_CTP_drag_drop.js
@@ -34,7 +34,7 @@ add_task(async function() {
   gNewWindow = gBrowser.replaceTabWithWindow(gBrowser.selectedTab);
 
   // XXX technically can't load fire before we get this call???
-  await waitForEvent(gNewWindow, "load", null, true);
+  await BrowserTestUtils.waitForEvent(gNewWindow, "load", true);
 
   await promisePopupNotification("click-to-play-plugins", gNewWindow.gBrowser.selectedBrowser);
 
diff --git a/browser/base/content/test/plugins/browser_CTP_nonplugins.js b/browser/base/content/test/plugins/browser_CTP_nonplugins.js
index 9c843daed30f..cfad122759e1 100644
--- a/browser/base/content/test/plugins/browser_CTP_nonplugins.js
+++ b/browser/base/content/test/plugins/browser_CTP_nonplugins.js
@@ -31,7 +31,7 @@ add_task(async function() {
   let popupNotification = PopupNotifications.getNotification("click-to-play-plugins", gBrowser.selectedBrowser);
   ok(popupNotification, "Test 1, Should have a click-to-play notification");
 
-  let pluginRemovedPromise = waitForEvent(gBrowser.selectedBrowser, "PluginRemoved", null, true, true);
+  let pluginRemovedPromise = BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "PluginRemoved", true, null, true);
   await ContentTask.spawn(gBrowser.selectedBrowser, {}, async function() {
     let plugin = content.document.getElementById("secondtestA");
     plugin.remove();
diff --git a/browser/base/content/test/plugins/browser_blocking.js b/browser/base/content/test/plugins/browser_blocking.js
index 6d4b096d076d..fc2070ffb9a6 100644
--- a/browser/base/content/test/plugins/browser_blocking.js
+++ b/browser/base/content/test/plugins/browser_blocking.js
@@ -70,7 +70,7 @@ add_task(async function() {
       "Test 18a, Plugin should have an update link");
   });
 
-  let promise = waitForEvent(gBrowser.tabContainer, "TabOpen", null, true);
+  let promise = BrowserTestUtils.waitForEvent(gBrowser.tabContainer, "TabOpen", true);
 
   await ContentTask.spawn(gTestBrowser, {}, async function() {
     let doc = content.document;
@@ -86,7 +86,7 @@ add_task(async function() {
   });
   await promise;
 
-  promise = waitForEvent(gBrowser.tabContainer, "TabClose", null, true);
+  promise = BrowserTestUtils.waitForEvent(gBrowser.tabContainer, "TabClose", true);
   gBrowser.removeCurrentTab();
   await promise;
 });
diff --git a/browser/base/content/test/plugins/browser_bug744745.js b/browser/base/content/test/plugins/browser_bug744745.js
index 81656c5d58fe..25eac6532f40 100644
--- a/browser/base/content/test/plugins/browser_bug744745.js
+++ b/browser/base/content/test/plugins/browser_bug744745.js
@@ -30,7 +30,7 @@ add_task(async function() {
 
   setTestPluginEnabledState(Ci.nsIPluginTag.STATE_CLICKTOPLAY, "Test Plug-in");
 
-  gTestBrowser.addEventListener("PluginBindingAttached", pluginBindingAttached, true, true);
+  BrowserTestUtils.addContentEventListener(gTestBrowser, "PluginBindingAttached", pluginBindingAttached, true, null, true);
 
   let testRoot = getRootDirectory(gTestPath).replace("chrome://mochitests/content/", "http://127.0.0.1:8888/");
   await promiseTabLoadEvent(gBrowser.selectedTab, testRoot + "plugin_bug744745.html");
diff --git a/browser/base/content/test/plugins/browser_bug820497.js b/browser/base/content/test/plugins/browser_bug820497.js
index 1c27b92ab477..d039ed423695 100644
--- a/browser/base/content/test/plugins/browser_bug820497.js
+++ b/browser/base/content/test/plugins/browser_bug820497.js
@@ -23,7 +23,7 @@ add_task(async function() {
   setTestPluginEnabledState(Ci.nsIPluginTag.STATE_CLICKTOPLAY, "Test Plug-in");
   setTestPluginEnabledState(Ci.nsIPluginTag.STATE_CLICKTOPLAY, "Second Test Plug-in");
 
-  gTestBrowser.addEventListener("PluginBindingAttached", function() { gNumPluginBindingsAttached++; }, true, true);
+  BrowserTestUtils.addContentEventListener(gTestBrowser, "PluginBindingAttached", function() { gNumPluginBindingsAttached++; }, true, null, true);
 
   await promiseTabLoadEvent(gBrowser.selectedTab, gTestRoot + "plugin_bug820497.html");
 
diff --git a/browser/base/content/test/plugins/browser_pageInfo_plugins.js b/browser/base/content/test/plugins/browser_pageInfo_plugins.js
index 5a4df02ddfaf..6d9f367ef652 100644
--- a/browser/base/content/test/plugins/browser_pageInfo_plugins.js
+++ b/browser/base/content/test/plugins/browser_pageInfo_plugins.js
@@ -9,12 +9,11 @@ var gSecondTestPermissionString = gPluginHost.getPermissionStringForType("applic
 
 function doOnPageLoad(url, continuation) {
   gNextTest = continuation;
-  gTestBrowser.addEventListener("load", pageLoad, true);
-  gTestBrowser.contentWindow.location = url;
+  BrowserTestUtils.browserLoaded(gTestBrowser).then(pageLoad);
+  gTestBrowser.loadURI(url);
 }
 
 function pageLoad() {
-  gTestBrowser.removeEventListener("load", pageLoad);
   // The plugin events are async dispatched and can come after the load event
   // This just allows the events to fire before we then go on to test the states
   executeSoon(gNextTest);
@@ -63,10 +62,10 @@ function test() {
 
 // The first test plugin is CtP and the second test plugin is enabled.
 function testPart1a() {
-  let testElement = gTestBrowser.contentDocument.getElementById("test");
+  let testElement = gTestBrowser.contentDocumentAsCPOW.getElementById("test");
   let objLoadingContent = testElement.QueryInterface(Ci.nsIObjectLoadingContent);
   ok(!objLoadingContent.activated, "part 1a: Test plugin should not be activated");
-  let secondtest = gTestBrowser.contentDocument.getElementById("secondtestA");
+  let secondtest = gTestBrowser.contentDocumentAsCPOW.getElementById("secondtestA");
   objLoadingContent = secondtest.QueryInterface(Ci.nsIObjectLoadingContent);
   ok(objLoadingContent.activated, "part 1a: Second Test plugin should be activated");
 
@@ -94,11 +93,11 @@ function testPart1b() {
 
 // Now, the Test plugin should be allowed, and the Test2 plugin should be CtP
 function testPart2() {
-  let testElement = gTestBrowser.contentDocument.getElementById("test").
+  let testElement = gTestBrowser.contentDocumentAsCPOW.getElementById("test").
     QueryInterface(Ci.nsIObjectLoadingContent);
   ok(testElement.activated, "part 2: Test plugin should be activated");
 
-  let secondtest = gTestBrowser.contentDocument.getElementById("secondtestA").
+  let secondtest = gTestBrowser.contentDocumentAsCPOW.getElementById("secondtestA").
     QueryInterface(Ci.nsIObjectLoadingContent);
   ok(!secondtest.activated, "part 2: Second Test plugin should not be activated");
   is(secondtest.pluginFallbackType, Ci.nsIObjectLoadingContent.PLUGIN_CLICK_TO_PLAY,
@@ -123,13 +122,13 @@ function testPart2() {
 
 // Now, all the things should be blocked
 function testPart3() {
-  let testElement = gTestBrowser.contentDocument.getElementById("test").
+  let testElement = gTestBrowser.contentDocumentAsCPOW.getElementById("test").
     QueryInterface(Ci.nsIObjectLoadingContent);
   ok(!testElement.activated, "part 3: Test plugin should not be activated");
   is(testElement.pluginFallbackType, Ci.nsIObjectLoadingContent.PLUGIN_DISABLED,
     "part 3: Test plugin should be marked as PLUGIN_DISABLED");
 
-  let secondtest = gTestBrowser.contentDocument.getElementById("secondtestA").
+  let secondtest = gTestBrowser.contentDocumentAsCPOW.getElementById("secondtestA").
     QueryInterface(Ci.nsIObjectLoadingContent);
 
   ok(!secondtest.activated, "part 3: Second Test plugin should not be activated");
diff --git a/browser/base/content/test/plugins/head.js b/browser/base/content/test/plugins/head.js
index 4c3c522a6c29..a7db23e992cf 100644
--- a/browser/base/content/test/plugins/head.js
+++ b/browser/base/content/test/plugins/head.js
@@ -44,28 +44,6 @@ function waitForMs(aMs) {
   });
 }
 
-function waitForEvent(subject, eventName, checkFn, useCapture, useUntrusted) {
-  return new Promise((resolve, reject) => {
-    subject.addEventListener(eventName, function listener(event) {
-      try {
-        if (checkFn && !checkFn(event)) {
-          return;
-        }
-        subject.removeEventListener(eventName, listener, useCapture);
-        resolve(event);
-      } catch (ex) {
-        try {
-          subject.removeEventListener(eventName, listener, useCapture);
-        } catch (ex2) {
-          // Maybe the provided object does not support removeEventListener.
-        }
-        reject(ex);
-      }
-    }, useCapture, useUntrusted);
-  });
-}
-
-
 /**
  * Waits for a load (or custom) event to finish in a given tab. If provided
  * load an uri into the tab.
diff --git a/browser/base/content/test/siteIdentity/browser_bug906190.js b/browser/base/content/test/siteIdentity/browser_bug906190.js
index 28bdfc1dfecb..e73e006b19a1 100644
--- a/browser/base/content/test/siteIdentity/browser_bug906190.js
+++ b/browser/base/content/test/siteIdentity/browser_bug906190.js
@@ -34,16 +34,19 @@ async function doTest(parentTabSpec, childTabSpec, testTaskFn, waitForMetaRefres
     await promiseReloaded;
 
     // Wait for the script in the page to update the contents of the test div.
-    let testDiv = content.document.getElementById("mctestdiv");
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    let testDiv = gBrowser.contentDocumentAsCPOW.getElementById("mctestdiv");
     await BrowserTestUtils.waitForCondition(
       () => testDiv.innerHTML == "Mixed Content Blocker disabled");
 
     // Add the link for the child tab to the page.
-    let mainDiv = content.document.createElement("div");
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    let mainDiv = gBrowser.contentDocumentAsCPOW.createElement("div");
     // eslint-disable-next-line no-unsanitized/property
     mainDiv.innerHTML =
       '<p><a id="linkToOpenInNewTab" href="' + childTabSpec + '">Link</a></p>';
-    content.document.body.appendChild(mainDiv);
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    gBrowser.contentDocumentAsCPOW.body.appendChild(mainDiv);
 
     // Execute the test in the child tabs with the two methods to open it.
     for (let openFn of [simulateCtrlClick, simulateContextMenuOpenInTab]) {
@@ -85,14 +88,9 @@ function simulateContextMenuOpenInTab(browser) {
 // from <xul:browser>s without a tab assigned. That are most likely browsers
 // that preload the new tab page.
 function waitForSomeTabToLoad() {
-  return new Promise(resolve => {
-    gBrowser.addEventListener("load", function onLoad(event) {
-      let tab = gBrowser._getTabForContentWindow(event.target.defaultView.top);
-      if (tab) {
-        gBrowser.removeEventListener("load", onLoad, true);
-        resolve();
-      }
-    }, true);
+  return BrowserTestUtils.firstBrowserLoaded(window, true, (browser) => {
+    let tab = gBrowser.getTabForBrowser(browser);
+    return !!tab;
   });
 }
 
@@ -121,7 +119,8 @@ add_task(async function test_same_origin() {
       activeLoaded: true, activeBlocked: false, passiveLoaded: false,
     });
 
-    is(content.document.getElementById("mctestdiv").innerHTML,
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    is(gBrowser.contentDocumentAsCPOW.getElementById("mctestdiv").innerHTML,
        "Mixed Content Blocker disabled", "OK: Executed mixed script");
   });
 });
@@ -142,7 +141,8 @@ add_task(async function test_different_origin() {
       activeLoaded: false, activeBlocked: true, passiveLoaded: false,
     });
 
-    is(content.document.getElementById("mctestdiv").innerHTML,
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    is(gBrowser.contentDocumentAsCPOW.getElementById("mctestdiv").innerHTML,
        "Mixed Content Blocker enabled", "OK: Blocked mixed script");
   });
 });
@@ -163,7 +163,8 @@ add_task(async function test_same_origin_metarefresh_same_origin() {
       activeLoaded: true, activeBlocked: false, passiveLoaded: false,
     });
 
-    is(content.document.getElementById("mctestdiv").innerHTML,
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    is(gBrowser.contentDocumentAsCPOW.getElementById("mctestdiv").innerHTML,
        "Mixed Content Blocker disabled", "OK: Executed mixed script");
   }, true);
 });
@@ -183,7 +184,8 @@ add_task(async function test_same_origin_metarefresh_different_origin() {
       activeLoaded: false, activeBlocked: true, passiveLoaded: false,
     });
 
-    is(content.document.getElementById("mctestdiv").innerHTML,
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    is(gBrowser.contentDocumentAsCPOW.getElementById("mctestdiv").innerHTML,
        "Mixed Content Blocker enabled", "OK: Blocked mixed script");
   }, true);
 });
@@ -203,7 +205,8 @@ add_task(async function test_same_origin_302redirect_same_origin() {
     ok(!gIdentityHandler._identityBox.classList.contains("mixedActiveBlocked"),
        "OK: Mixed Content is NOT being blocked");
 
-    is(content.document.getElementById("mctestdiv").innerHTML,
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    is(gBrowser.contentDocumentAsCPOW.getElementById("mctestdiv").innerHTML,
        "Mixed Content Blocker disabled", "OK: Executed mixed script");
   });
 });
@@ -223,7 +226,8 @@ add_task(async function test_same_origin_302redirect_different_origin() {
       activeLoaded: false, activeBlocked: true, passiveLoaded: false,
     });
 
-    is(content.document.getElementById("mctestdiv").innerHTML,
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    is(gBrowser.contentDocumentAsCPOW.getElementById("mctestdiv").innerHTML,
        "Mixed Content Blocker enabled", "OK: Blocked mixed script");
   });
 });
diff --git a/browser/base/content/test/siteIdentity/browser_identity_UI.js b/browser/base/content/test/siteIdentity/browser_identity_UI.js
index c733e314e564..5d884e9b9a7c 100644
--- a/browser/base/content/test/siteIdentity/browser_identity_UI.js
+++ b/browser/base/content/test/siteIdentity/browser_identity_UI.js
@@ -7,7 +7,7 @@ function test() {
   ok(gIdentityHandler, "gIdentityHandler should exist");
 
   BrowserTestUtils.openNewForegroundTab(gBrowser, "about:blank", true).then(() => {
-    gBrowser.selectedBrowser.addEventListener("load", checkResult, true);
+    BrowserTestUtils.addContentEventListener(gBrowser.selectedBrowser, "load", checkResult, true);
     nextTest();
   });
 }
@@ -79,7 +79,6 @@ function nextTest() {
     }
 
     if (gCurrentTestIndex == -1) {
-      gBrowser.selectedBrowser.removeEventListener("load", checkResult, true);
       gBrowser.removeCurrentTab();
       finish();
       return;
@@ -123,8 +122,8 @@ function nextTest() {
   }
 }
 
-function checkResult(event) {
-  if (event.target.URL == "about:blank")
+function checkResult() {
+  if (gBrowser.selectedBrowser.currentURI.spec == "about:blank")
     return;
 
   // Sanity check other values, and the value of gIdentityHandler.getEffectiveHost()
diff --git a/browser/base/content/test/urlbar/browser_urlbarHashChangeProxyState.js b/browser/base/content/test/urlbar/browser_urlbarHashChangeProxyState.js
index 21a0f52f3085..d43b12856624 100644
--- a/browser/base/content/test/urlbar/browser_urlbarHashChangeProxyState.js
+++ b/browser/base/content/test/urlbar/browser_urlbarHashChangeProxyState.js
@@ -92,7 +92,7 @@ add_task(async function() {
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, "about:newtab", false);
   // NB: CPOW usage because new tab pages can be preloaded, in which case no
   // load events fire.
-  await BrowserTestUtils.waitForCondition(() => !tab.linkedBrowser.contentDocument.hidden);
+  await BrowserTestUtils.waitForCondition(() => !tab.linkedBrowser.contentDocumentAsCPOW.hidden);
   let url = "https://example.org/browser/browser/base/content/test/urlbar/dummy_page.html#foo";
   gURLBar.value = url;
   gURLBar.select();
diff --git a/browser/base/content/test/urlbar/browser_urlbarKeepStateAcrossTabSwitches.js b/browser/base/content/test/urlbar/browser_urlbarKeepStateAcrossTabSwitches.js
index 86d092e8acdd..6527d7753efa 100644
--- a/browser/base/content/test/urlbar/browser_urlbarKeepStateAcrossTabSwitches.js
+++ b/browser/base/content/test/urlbar/browser_urlbarKeepStateAcrossTabSwitches.js
@@ -7,9 +7,11 @@
 add_task(async function() {
   let input = "i-definitely-dont-exist.example.com";
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, "about:blank", false);
+  let browser = tab.linkedBrowser;
   // NB: CPOW usage because new tab pages can be preloaded, in which case no
   // load events fire.
-  await BrowserTestUtils.waitForCondition(() => !tab.linkedBrowser.contentDocument.hidden);
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
+  await BrowserTestUtils.waitForCondition(() => browser.contentDocumentAsCPOW && !browser.contentDocumentAsCPOW.hidden);
   let errorPageLoaded = BrowserTestUtils.waitForErrorPage(tab.linkedBrowser);
   gURLBar.value = input;
   gURLBar.select();
@@ -30,9 +32,11 @@ add_task(async function() {
   let input = "To be or not to be-that is the question";
   await SpecialPowers.pushPrefEnv({set: [["keyword.enabled", false]]});
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, "about:blank", false);
+  let browser = tab.linkedBrowser;
   // NB: CPOW usage because new tab pages can be preloaded, in which case no
   // load events fire.
-  await BrowserTestUtils.waitForCondition(() => !tab.linkedBrowser.contentDocument.hidden);
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
+  await BrowserTestUtils.waitForCondition(() => browser.contentDocumentAsCPOW && !browser.contentDocumentAsCPOW.hidden);
   let errorPageLoaded = BrowserTestUtils.waitForErrorPage(tab.linkedBrowser);
   gURLBar.value = input;
   gURLBar.select();
diff --git a/browser/components/contextualidentity/test/browser/browser_usercontext.js b/browser/components/contextualidentity/test/browser/browser_usercontext.js
index 27fb560369a2..d7787a82fc8c 100644
--- a/browser/components/contextualidentity/test/browser/browser_usercontext.js
+++ b/browser/components/contextualidentity/test/browser/browser_usercontext.js
@@ -70,7 +70,7 @@ add_task(async function test() {
 
     // get the title
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    let title = browser.contentDocument.title.trim().split("|");
+    let title = browser.contentDocumentAsCPOW.title.trim().split("|");
 
     // check each item in the title and validate it meets expectatations
     for (let part of title) {
diff --git a/browser/components/customizableui/test/browser_synced_tabs_menu.js b/browser/components/customizableui/test/browser_synced_tabs_menu.js
index 132e14381f63..be902989e4d2 100644
--- a/browser/components/customizableui/test/browser_synced_tabs_menu.js
+++ b/browser/components/customizableui/test/browser_synced_tabs_menu.js
@@ -169,9 +169,7 @@ add_task(async function() {
   is(gBrowser.tabs.length, 2, "there's a new tab");
   await new Promise(resolve => {
     if (gBrowser.selectedBrowser.currentURI.spec == "about:blank") {
-      gBrowser.selectedBrowser.addEventListener("load", function(e) {
-        resolve();
-      }, {capture: true, once: true});
+      BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(resolve);
       return;
     }
     // the new tab has already transitioned away from about:blank so we
diff --git a/browser/components/customizableui/test/head.js b/browser/components/customizableui/test/head.js
index e2d7a9323ef7..10e13be5e436 100644
--- a/browser/components/customizableui/test/head.js
+++ b/browser/components/customizableui/test/head.js
@@ -401,7 +401,7 @@ function promiseTabHistoryNavigation(aDirection = -1, aConditionFn) {
     }
     gBrowser.addEventListener("pageshow", listener, true);
 
-    content.history.go(aDirection);
+    gBrowser.contentWindowAsCPOW.history.go(aDirection);
 
   });
 }
diff --git a/browser/components/extensions/test/browser/head.js b/browser/components/extensions/test/browser/head.js
index 6647ce662e0a..cd71bcd06857 100644
--- a/browser/components/extensions/test/browser/head.js
+++ b/browser/components/extensions/test/browser/head.js
@@ -289,7 +289,7 @@ async function openContextMenuInSidebar(selector = "body") {
 async function openContextMenuInFrame(frameId) {
   let contentAreaContextMenu = document.getElementById("contentAreaContextMenu");
   let popupShownPromise = BrowserTestUtils.waitForEvent(contentAreaContextMenu, "popupshown");
-  let doc = gBrowser.selectedBrowser.contentDocument;
+  let doc = gBrowser.selectedBrowser.contentDocumentAsCPOW;
   let frame = doc.getElementById(frameId);
   EventUtils.synthesizeMouseAtCenter(frame.contentDocument.body, {type: "contextmenu"}, frame.contentWindow);
   await popupShownPromise;
diff --git a/browser/components/preferences/in-content/tests/browser_siteData.js b/browser/components/preferences/in-content/tests/browser_siteData.js
index 8419af4a188e..f8cec1f243b4 100644
--- a/browser/components/preferences/in-content/tests/browser_siteData.js
+++ b/browser/components/preferences/in-content/tests/browser_siteData.js
@@ -140,7 +140,7 @@ add_task(async function() {
   // Open a test site which would save into quota manager
   await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_QUOTA_USAGE_URL);
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  await waitForEvent(gBrowser.selectedBrowser.contentWindow, "test-indexedDB-done");
+  await waitForEvent(gBrowser.selectedBrowser.contentWindowAsCPOW, "test-indexedDB-done");
   await BrowserTestUtils.removeTab(gBrowser.selectedTab);
 
   let updatedPromise = promiseSiteDataManagerSitesUpdated();
@@ -224,7 +224,7 @@ add_task(async function() {
 
   await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_QUOTA_USAGE_URL);
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  await waitForEvent(gBrowser.selectedBrowser.contentWindow, "test-indexedDB-done");
+  await waitForEvent(gBrowser.selectedBrowser.contentWindowAsCPOW, "test-indexedDB-done");
   await BrowserTestUtils.removeTab(gBrowser.selectedTab);
 
   await openPreferencesViaOpenPreferencesAPI("privacy", { leaveOpen: true });
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_cache.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_cache.js
index 9c77d2baeac0..b85b4b7f027a 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_cache.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_cache.js
@@ -103,7 +103,7 @@ function get_cache_for_private_window() {
       win.gBrowser.selectedTab = tab;
       let newTabBrowser = win.gBrowser.getBrowserForTab(tab);
 
-      newTabBrowser.addEventListener("load", function() {
+      BrowserTestUtils.browserLoaded(newTabBrowser).then(function() {
         executeSoon(function() {
 
           getStorageEntryCount("private", function(nrEntriesP) {
@@ -119,7 +119,7 @@ function get_cache_for_private_window() {
             });
           });
         });
-      }, {capture: true, once: true});
+      });
     });
   });
 }
diff --git a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js
index c721e2fa9a27..1c0e5fef0bbe 100644
--- a/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_ui.js
@@ -14,7 +14,7 @@ function test() {
   let cmd;
 
   function doTest(aIsPrivateMode, aWindow, aCallback) {
-    aWindow.gBrowser.selectedBrowser.addEventListener("load", function() {
+    BrowserTestUtils.browserLoaded(aWindow.gBrowser.selectedBrowser).then(function() {
       ok(aWindow.gPrivateBrowsingUI, "The gPrivateBrowsingUI object exists");
 
       pbMenuItem = aWindow.document.getElementById("menu_newPrivateWindow");
@@ -30,7 +30,7 @@ function test() {
         aIsPrivateMode + ")");
 
       aCallback();
-    }, {capture: true, once: true});
+    });
 
     aWindow.gBrowser.selectedBrowser.loadURI(testURI);
   }
diff --git a/browser/components/search/test/browser_aboutSearchReset.js b/browser/components/search/test/browser_aboutSearchReset.js
index f748c22111ca..60ff62fcf480 100644
--- a/browser/components/search/test/browser_aboutSearchReset.js
+++ b/browser/components/search/test/browser_aboutSearchReset.js
@@ -65,7 +65,7 @@ var gTests = [
 
     let loadPromise = promiseStoppedLoad(expectedURL);
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    gBrowser.contentDocument.getElementById("searchResetKeepCurrent").click();
+    gBrowser.contentDocumentAsCPOW.getElementById("searchResetKeepCurrent").click();
     await loadPromise;
 
     is(engine, Services.search.currentEngine,
@@ -84,7 +84,7 @@ var gTests = [
     let currentEngine = Services.search.currentEngine;
     let originalEngine = Services.search.originalDefaultEngine;
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    let doc = gBrowser.contentDocument;
+    let doc = gBrowser.contentDocumentAsCPOW;
     let defaultEngineSpan = doc.getElementById("defaultEngine");
     is(defaultEngineSpan.textContent, originalEngine.name,
        "the name of the original default engine is displayed");
@@ -117,7 +117,7 @@ var gTests = [
                                                      false,
                                                      "about:preferences#search");
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    gBrowser.contentDocument.getElementById("linkSettingsPage").click();
+    gBrowser.contentDocumentAsCPOW.getElementById("linkSettingsPage").click();
     await loadPromise;
 
     checkTelemetryRecords(TELEMETRY_RESULT_ENUM.OPENED_SETTINGS);
diff --git a/browser/components/sessionstore/test/browser_739531.js b/browser/components/sessionstore/test/browser_739531.js
index fe60cde4f159..14c81a985ec2 100644
--- a/browser/components/sessionstore/test/browser_739531.js
+++ b/browser/components/sessionstore/test/browser_739531.js
@@ -13,11 +13,13 @@ function test() {
 
   let loadCount = 0;
   let tab = BrowserTestUtils.addTab(gBrowser, testURL);
-  tab.linkedBrowser.addEventListener("load", function onLoad(aEvent) {
+
+  let removeFunc;
+  removeFunc = BrowserTestUtils.addContentEventListener(tab.linkedBrowser, "load", function onLoad(aEvent) {
     // make sure both the page and the frame are loaded
     if (++loadCount < 2)
       return;
-    tab.linkedBrowser.removeEventListener("load", onLoad, true);
+    removeFunc();
 
     // executeSoon to allow the JS to execute on the page
     executeSoon(function() {
diff --git a/browser/components/shell/test/browser_1119088.js b/browser/components/shell/test/browser_1119088.js
index ed70cdb5b7a9..ffe159f295c8 100644
--- a/browser/components/shell/test/browser_1119088.js
+++ b/browser/components/shell/test/browser_1119088.js
@@ -4,8 +4,6 @@ let NS_MAC_USER_LIB_DIR = "ULibDir";
 
 
 function onPageLoad() {
-  gBrowser.selectedBrowser.removeEventListener("load", onPageLoad, true);
-
   let dirSvc = Cc["@mozilla.org/file/directory_service;1"].
                getService(Ci.nsIDirectoryServiceProvider);
 
@@ -34,7 +32,8 @@ function onPageLoad() {
   let shell = Cc["@mozilla.org/browser/shell-service;1"].
               getService(Ci.nsIShellService);
 
-  let image = content.document.images[0];
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
+  let image = gBrowser.contentDocumentAsCPOW.images[0];
   shell.setDesktopBackground(image, 0, "logo.png");
 
   setTimeout(function() {
@@ -59,8 +58,8 @@ function onPageLoad() {
 
 function test() {
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", onPageLoad, true);
-  content.location = "about:logo";
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(onPageLoad, false, "about:logo");
+  gBrowser.loadURI("about:logo");
 
   waitForExplicitFinish();
 }
diff --git a/browser/components/translation/test/browser_translation_bing.js b/browser/components/translation/test/browser_translation_bing.js
index 0c0ddb776a76..77b79049756f 100644
--- a/browser/components/translation/test/browser_translation_bing.js
+++ b/browser/components/translation/test/browser_translation_bing.js
@@ -121,12 +121,9 @@ function promiseTestPageLoad(url) {
   return new Promise(resolve => {
     let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, url);
     let browser = gBrowser.selectedBrowser;
-    browser.addEventListener("load", function listener() {
-      if (browser.currentURI.spec == "about:blank")
-        return;
+    BrowserTestUtils.browserLoaded(browser, false, (loadurl) => loadurl != "about:blank").then(() => {
       info("Page loaded: " + browser.currentURI.spec);
-      browser.removeEventListener("load", listener, true);
       resolve(tab);
-    }, true);
+    });
   });
 }
diff --git a/browser/components/translation/test/browser_translation_exceptions.js b/browser/components/translation/test/browser_translation_exceptions.js
index dbda0e51ed9b..89abd8fa93bd 100644
--- a/browser/components/translation/test/browser_translation_exceptions.js
+++ b/browser/components/translation/test/browser_translation_exceptions.js
@@ -21,7 +21,7 @@ function test() {
     gBrowser.removeTab(tab);
     Services.prefs.clearUserPref(kShowUIPref);
   });
-  tab.linkedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => {
     (async function() {
       for (let testCase of gTests) {
         info(testCase.desc);
@@ -31,9 +31,9 @@ function test() {
      ok(false, "Unexpected Exception: " + ex);
      finish();
     });
-   }, {capture: true, once: true});
+   });
 
-  content.location = "http://example.com/";
+  gBrowser.selectedBrowser.loadURI("http://example.com/");
 }
 
 function getLanguageExceptions() {
diff --git a/browser/components/translation/test/browser_translation_infobar.js b/browser/components/translation/test/browser_translation_infobar.js
index 3efa7b0dca86..4fd03a9fa0f4 100644
--- a/browser/components/translation/test/browser_translation_infobar.js
+++ b/browser/components/translation/test/browser_translation_infobar.js
@@ -77,7 +77,7 @@ function test() {
   Services.prefs.setBoolPref(kShowUIPref, true);
   let tab = BrowserTestUtils.addTab(gBrowser);
   gBrowser.selectedTab = tab;
-  tab.linkedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => {
     TranslationStub.browser = gBrowser.selectedBrowser;
     registerCleanupFunction(function() {
       gBrowser.removeTab(tab);
@@ -86,9 +86,9 @@ function test() {
     run_tests(() => {
       finish();
     });
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/plain,test page";
+  gBrowser.selectedBrowser.loadURI("data:text/plain,test page");
 }
 
 function checkURLBarIcon(aExpectTranslated = false) {
diff --git a/browser/components/translation/test/browser_translation_yandex.js b/browser/components/translation/test/browser_translation_yandex.js
index 5f4b591f3331..7e68397768a8 100644
--- a/browser/components/translation/test/browser_translation_yandex.js
+++ b/browser/components/translation/test/browser_translation_yandex.js
@@ -116,13 +116,10 @@ function promiseTestPageLoad(url) {
   return new Promise(resolve => {
     let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, url);
     let browser = gBrowser.selectedBrowser;
-    browser.addEventListener("load", function listener() {
-      if (browser.currentURI.spec == "about:blank")
-        return;
+    BrowserTestUtils.browserLoaded(browser, false, (loadurl) => loadurl != "about:blank").then(() => {
       info("Page loaded: " + browser.currentURI.spec);
-      browser.removeEventListener("load", listener, true);
       resolve(tab);
-    }, true);
+    });
   });
 }
 
diff --git a/browser/components/uitour/test/head.js b/browser/components/uitour/test/head.js
index 622fb6bef44c..7252658c8420 100644
--- a/browser/components/uitour/test/head.js
+++ b/browser/components/uitour/test/head.js
@@ -263,7 +263,7 @@ function loadUITourTestPage(callback, host = "https://example.org/") {
   gTestTab = BrowserTestUtils.addTab(gBrowser, url);
   gBrowser.selectedTab = gTestTab;
 
-  gTestTab.linkedBrowser.addEventListener("load", function() {
+  BrowserTestUtils.browserLoaded(gTestTab.linkedBrowser).then(() => {
     if (gMultiProcessBrowser) {
       // When e10s is enabled, make gContentAPI and gContentWindow proxies which has every property
       // return a function which calls the method of the same name on
@@ -352,7 +352,7 @@ function loadUITourTestPage(callback, host = "https://example.org/") {
     }
 
     waitForFocus(callback, gTestTab.linkedBrowser);
-  }, {capture: true, once: true});
+  });
 }
 
 // Wrapper for UITourTest to be used by add_task tests.
diff --git a/browser/extensions/onboarding/test/browser/browser_onboarding_skip_tour.js b/browser/extensions/onboarding/test/browser/browser_onboarding_skip_tour.js
index 7b2f1ec798e7..0388dc4056ca 100644
--- a/browser/extensions/onboarding/test/browser/browser_onboarding_skip_tour.js
+++ b/browser/extensions/onboarding/test/browser/browser_onboarding_skip_tour.js
@@ -37,7 +37,7 @@ add_task(async function test_hide_skip_button_via_perf() {
   await promiseOnboardingOverlayOpened(tab.linkedBrowser);
 
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  ok(!content.document.querySelector("#onboarding-skip-tour-button"), "should not render the skip button");
+  ok(!gBrowser.contentDocumentAsCPOW.querySelector("#onboarding-skip-tour-button"), "should not render the skip button");
 
   await BrowserTestUtils.removeTab(tab);
 });
diff --git a/browser/extensions/onboarding/test/browser/browser_onboarding_tourset.js b/browser/extensions/onboarding/test/browser/browser_onboarding_tourset.js
index 28ed8db582df..0ab96b321dd2 100644
--- a/browser/extensions/onboarding/test/browser/browser_onboarding_tourset.js
+++ b/browser/extensions/onboarding/test/browser/browser_onboarding_tourset.js
@@ -14,7 +14,7 @@ add_task(async function test_onboarding_default_new_tourset() {
   await promiseOnboardingOverlayOpened(tab.linkedBrowser);
 
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  let doc = content && content.document;
+  let doc = gBrowser.contentDocumentAsCPOW;
   let doms = doc.querySelectorAll(".onboarding-tour-item");
   is(doms.length, TOUR_IDs.length, "has exact tour numbers");
   doms.forEach((dom, idx) => {
@@ -45,7 +45,7 @@ add_task(async function test_onboarding_custom_new_tourset() {
   await promiseOnboardingOverlayOpened(tab.linkedBrowser);
 
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  let doc = content && content.document;
+  let doc = gBrowser.contentDocumentAsCPOW;
   let doms = doc.querySelectorAll(".onboarding-tour-item");
   is(doms.length, CUSTOM_NEW_TOURs.length, "has exact tour numbers");
   doms.forEach((dom, idx) => {
@@ -75,7 +75,7 @@ add_task(async function test_onboarding_custom_update_tourset() {
   await promiseOnboardingOverlayOpened(tab.linkedBrowser);
 
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  let doc = content && content.document;
+  let doc = gBrowser.contentDocumentAsCPOW;
   let doms = doc.querySelectorAll(".onboarding-tour-item");
   is(doms.length, CUSTOM_UPDATE_TOURs.length, "has exact tour numbers");
   doms.forEach((dom, idx) => {
diff --git a/browser/extensions/shield-recipe-client/test/browser/browser_about_studies.js b/browser/extensions/shield-recipe-client/test/browser/browser_about_studies.js
index a70b7595fcf3..699d51574fa3 100644
--- a/browser/extensions/shield-recipe-client/test/browser/browser_about_studies.js
+++ b/browser/extensions/shield-recipe-client/test/browser/browser_about_studies.js
@@ -15,7 +15,7 @@ decorate_task(
   withAboutStudies,
   async function testAboutStudiesWorks(browser) {
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    ok(browser.contentDocument.getElementById("app"), "App element was found");
+    ok(browser.contentDocumentAsCPOW.getElementById("app"), "App element was found");
   }
 );
 
@@ -43,6 +43,8 @@ decorate_task(
 decorate_task(
   withAboutStudies,
   async function testUpdatePreferencesNewOrganization(browser) {
+    let loadPromise = BrowserTestUtils.firstBrowserLoaded(window);
+
     // We have to use gBrowser instead of browser in most spots since we're
     // dealing with a new tab outside of the about:studies tab.
     const tab = await BrowserTestUtils.switchTab(gBrowser, () => {
@@ -51,13 +53,9 @@ decorate_task(
       });
     });
 
-    /* eslint-disable mozilla/no-cpows-in-tests */
-    if (gBrowser.contentDocument.readyState !== "complete") {
-      await BrowserTestUtils.waitForEvent(gBrowser.contentWindow, "load");
-    }
+    await loadPromise;
 
-    const location = gBrowser.contentWindow.location.href;
-    /* eslint-enable mozilla/no-cpows-in-tests */
+    const location = gBrowser.currentURI.spec;
     is(
       location,
       "about:preferences#privacy",
diff --git a/browser/modules/test/browser/browser_UsageTelemetry_content_aboutHome.js b/browser/modules/test/browser/browser_UsageTelemetry_content_aboutHome.js
index 339f9021fd5e..3d271514b742 100644
--- a/browser/modules/test/browser/browser_UsageTelemetry_content_aboutHome.js
+++ b/browser/modules/test/browser/browser_UsageTelemetry_content_aboutHome.js
@@ -53,12 +53,7 @@ add_task(async function test_abouthome_simpleQuery() {
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
 
   info("Setup waiting for AboutHomeLoadSnippetsCompleted.");
-  let promiseAboutHomeLoaded = new Promise(resolve => {
-    tab.linkedBrowser.addEventListener("AboutHomeLoadSnippetsCompleted", function loadListener(event) {
-      tab.linkedBrowser.removeEventListener("AboutHomeLoadSnippetsCompleted", loadListener, true);
-      resolve();
-    }, true, true);
-  });
+  let promiseAboutHomeLoaded = BrowserTestUtils.waitForContentEvent(tab.linkedBrowser, "AboutHomeLoadSnippetsCompleted", true, null, true);
 
   info("Load about:home.");
   tab.linkedBrowser.loadURI("about:home");
@@ -105,12 +100,7 @@ add_task(async function test_abouthome_activitystream_simpleQuery() {
   let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser);
 
   info("Setup waiting for search input to initialise.");
-  let promiseAboutHomeSearchLoaded = new Promise(resolve => {
-    tab.linkedBrowser.addEventListener("ContentSearchClient", function loadListener(event) {
-      tab.linkedBrowser.removeEventListener("ContentSearchClient", loadListener, true);
-      executeSoon(resolve);
-    }, true, true);
-  });
+  let promiseAboutHomeSearchLoaded = BrowserTestUtils.waitForContentEvent(tab.linkedBrowser, "ContentSearchClient", true, null, true).then(() => false);
 
   info("Load about:home.");
   tab.linkedBrowser.loadURI("about:home");
diff --git a/devtools/client/commandline/test/helpers.js b/devtools/client/commandline/test/helpers.js
index 83ee5c946d1d..911e78500f9c 100644
--- a/devtools/client/commandline/test/helpers.js
+++ b/devtools/client/commandline/test/helpers.js
@@ -132,7 +132,7 @@ var { helpers, assert } = (function () {
     options.browser = tabbrowser.getBrowserForTab(options.tab);
     options.target = TargetFactory.forTab(options.tab);
 
-    var loaded = helpers.listenOnce(options.browser, "load", true).then(function (ev) {
+    var loaded = BrowserTestUtils.browserLoaded(options.browser).then(function () {
       var reply = callback.call(null, options);
 
       return Promise.resolve(reply).catch(function (error) {
@@ -149,7 +149,7 @@ var { helpers, assert } = (function () {
       });
     });
 
-    options.browser.contentWindow.location = url;
+    options.browser.loadURI(url);
     return loaded;
   };
 
diff --git a/devtools/client/debugger/test/mochitest/browser_dbg_WorkerActor.attach.js b/devtools/client/debugger/test/mochitest/browser_dbg_WorkerActor.attach.js
index 654a8f809a0c..61453bfbc2e2 100644
--- a/devtools/client/debugger/test/mochitest/browser_dbg_WorkerActor.attach.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg_WorkerActor.attach.js
@@ -45,7 +45,7 @@ function test() {
     is(workerClient2.isClosed, false, "worker in tab 2 should not be closed");
 
     executeSoon(() => {
-      tab.linkedBrowser.contentWindow.history.back();
+      tab.linkedBrowser.goBack();
     });
     yield waitForWorkerClose(workerClient2);
     is(workerClient2.isClosed, true, "worker in tab 2 should be closed");
diff --git a/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-01.js b/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-01.js
index f876cda93322..e75ebea23da9 100644
--- a/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-01.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-01.js
@@ -90,7 +90,7 @@ function testEventListeners(aThreadClient) {
         ok(node, "There is a node property.");
         ok(node.object, "There is a node object property.");
         ok(node.selector == "window" ||
-          content.document.querySelectorAll(node.selector).length == 1,
+          gBrowser.contentDocumentAsCPOW.querySelectorAll(node.selector).length == 1,
           "The node property is a unique CSS selector.");
 
         let func = l.function;
diff --git a/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-02.js b/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-02.js
index 5569306e639b..50dd240f1e04 100644
--- a/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-02.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg_event-listeners-02.js
@@ -89,7 +89,7 @@ function testEventListeners(aThreadClient) {
         ok(node, "There is a node property.");
         ok(node.object, "There is a node object property.");
         ok(node.selector == "window" ||
-          content.document.querySelectorAll(node.selector).length == 1,
+          gBrowser.contentDocumentAsCPOW.querySelectorAll(node.selector).length == 1,
           "The node property is a unique CSS selector.");
 
         let func = l.function;
diff --git a/devtools/client/debugger/test/mochitest/browser_dbg_sources-contextmenu-02.js b/devtools/client/debugger/test/mochitest/browser_dbg_sources-contextmenu-02.js
index 41320141a196..8845c29be817 100644
--- a/devtools/client/debugger/test/mochitest/browser_dbg_sources-contextmenu-02.js
+++ b/devtools/client/debugger/test/mochitest/browser_dbg_sources-contextmenu-02.js
@@ -43,10 +43,10 @@ function test() {
       gBrowser.tabContainer.addEventListener("TabOpen", function (e) {
         ok(true, "A new tab loaded");
 
-        gBrowser.addEventListener("DOMContentLoaded", function (e) {
+        BrowserTestUtils.waitForContentEvent(e.target.linkedBrowser, "DOMContentLoaded").then(function () {
           // Pass along the new tab's URI.
           resolve(gBrowser.currentURI.spec);
-        }, {once: true});
+        });
       }, {once: true});
     });
   }
diff --git a/devtools/client/debugger/test/mochitest/head.js b/devtools/client/debugger/test/mochitest/head.js
index 903aa6a24d7f..26a863715a6d 100644
--- a/devtools/client/debugger/test/mochitest/head.js
+++ b/devtools/client/debugger/test/mochitest/head.js
@@ -567,7 +567,7 @@ let initDebugger = Task.async(function*(urlOrTab, options) {
   }
   info("Debugee tab added successfully: " + urlOrTab);
 
-  let debuggee = tab.linkedBrowser.contentWindow.wrappedJSObject;
+  let debuggee = tab.linkedBrowser.contentWindowAsCPOW.wrappedJSObject;
   let target = TargetFactory.forTab(tab);
 
   let toolbox = yield gDevTools.showToolbox(target, "jsdebugger");
diff --git a/devtools/client/framework/test/browser_keybindings_01.js b/devtools/client/framework/test/browser_keybindings_01.js
index 6cb4ab650218..a28613968d27 100644
--- a/devtools/client/framework/test/browser_keybindings_01.js
+++ b/devtools/client/framework/test/browser_keybindings_01.js
@@ -28,7 +28,7 @@ function test()
   let keysetMap = { };
 
   addTab(TEST_URL).then(function () {
-    doc = content.document;
+    doc = gBrowser.contentDocumentAsCPOW;
     node = doc.querySelector("h1");
     waitForFocus(setupKeyBindingsTest);
   });
diff --git a/devtools/client/framework/test/browser_toolbox_options_enable_serviceworkers_testing.js b/devtools/client/framework/test/browser_toolbox_options_enable_serviceworkers_testing.js
index 3273f439514e..5d9b0e2cb010 100644
--- a/devtools/client/framework/test/browser_toolbox_options_enable_serviceworkers_testing.js
+++ b/devtools/client/framework/test/browser_toolbox_options_enable_serviceworkers_testing.js
@@ -83,15 +83,10 @@ function toggleServiceWorkersTestingCheckbox() {
 }
 
 function reload() {
-  let deferred = defer();
-
-  gBrowser.selectedBrowser.addEventListener("load", function onLoad(evt) {
-    gBrowser.selectedBrowser.removeEventListener(evt.type, onLoad, true);
-    deferred.resolve();
-  }, true);
+  let promise = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
 
   executeInContent("devtools:test:reload", {}, {}, false);
-  return deferred.promise;
+  return promise;
 }
 
 function testRegisterSuccesses(data) {
diff --git a/devtools/client/framework/test/shared-head.js b/devtools/client/framework/test/shared-head.js
index c9cb21d2f1bc..cf85b36b8c03 100644
--- a/devtools/client/framework/test/shared-head.js
+++ b/devtools/client/framework/test/shared-head.js
@@ -88,7 +88,7 @@ const ConsoleObserver = {
   observe: function (subject, topic, data) {
     let message = subject.wrappedJSObject.arguments[0];
 
-    if (/Failed propType/.test(message)) {
+    if (message && /Failed propType/.test(message.toString())) {
       ok(false, message);
     }
   }
diff --git a/devtools/client/inspector/markup/test/browser_markup_load_01.js b/devtools/client/inspector/markup/test/browser_markup_load_01.js
index c0fe2be0c019..ad5cf8313816 100644
--- a/devtools/client/inspector/markup/test/browser_markup_load_01.js
+++ b/devtools/client/inspector/markup/test/browser_markup_load_01.js
@@ -74,9 +74,5 @@ function* chooseWithInspectElementContextMenu(selector, tab) {
 }
 
 function waitForLinkedBrowserEvent(tab, event) {
-  return new Promise(resolve => {
-    tab.linkedBrowser.addEventListener(event, function () {
-      resolve();
-    }, {capture: true, once: true});
-  });
+  return BrowserTestUtils.waitForContentEvent(tab.linkedBrowser, event, true);
 }
diff --git a/devtools/client/inspector/test/browser_inspector_highlighter-05.js b/devtools/client/inspector/test/browser_inspector_highlighter-05.js
index 479e596c8a09..59caa789152b 100644
--- a/devtools/client/inspector/test/browser_inspector_highlighter-05.js
+++ b/devtools/client/inspector/test/browser_inspector_highlighter-05.js
@@ -50,7 +50,7 @@ add_task(function* () {
   info("Open the inspector to a blank page.");
   let { inspector, tab, testActor } = yield openInspectorForURL("about:blank");
 
-  let pageLoaded = waitForPageLoad(tab);
+  let pageLoaded = BrowserTestUtils.browserLoaded(tab.linkedBrowser);
 
   info("Navigate to the test url and waiting for the page to be loaded.");
   yield navigateTo(inspector, TEST_URL);
@@ -64,7 +64,3 @@ add_task(function* () {
   is(yield testActor.isHighlighting(), true,
     "Box Model highlighter is working as expected.");
 });
-
-const waitForPageLoad = (tab) => new Promise(resolve => {
-  tab.linkedBrowser.addEventListener("load", resolve, {capture: true, once: true});
-});
diff --git a/devtools/client/inspector/test/browser_inspector_startup.js b/devtools/client/inspector/test/browser_inspector_startup.js
index 74f7006914a1..edd144f9501e 100644
--- a/devtools/client/inspector/test/browser_inspector_startup.js
+++ b/devtools/client/inspector/test/browser_inspector_startup.js
@@ -69,11 +69,7 @@ add_task(function* () {
 });
 
 function waitForLinkedBrowserEvent(tab, event) {
-  return new Promise(resolve => {
-    tab.linkedBrowser.addEventListener(event, function () {
-      resolve();
-    }, {capture: true, once: true});
-  });
+  return BrowserTestUtils.waitForContentEvent(tab.linkedBrowser, event, true);
 }
 
 function contentReadyState(tab) {
diff --git a/devtools/client/responsive.html/test/browser/browser_window_close.js b/devtools/client/responsive.html/test/browser/browser_window_close.js
index 29d9d1e344df..4d127f227ede 100644
--- a/devtools/client/responsive.html/test/browser/browser_window_close.js
+++ b/devtools/client/responsive.html/test/browser/browser_window_close.js
@@ -9,7 +9,7 @@ add_task(function* () {
   let newWindow = yield newWindowPromise;
 
   newWindow.focus();
-  yield once(newWindow.gBrowser, "load", true);
+  yield BrowserTestUtils.browserLoaded(newWindow.gBrowser.selectedBrowser);
 
   let tab = newWindow.gBrowser.selectedTab;
   yield openRDM(tab);
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_browser_last_window_closing.js b/devtools/client/scratchpad/test/browser_scratchpad_browser_last_window_closing.js
index 811a07054972..1736db477238 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_browser_last_window_closing.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_browser_last_window_closing.js
@@ -14,11 +14,11 @@ function test()
   CloseObserver.init();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,<p>test browser last window closing</p>";
+  gBrowser.loadURI("data:text/html;charset=utf8,<p>test browser last window closing</p>");
 }
 
 
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_chrome_context_pref.js b/devtools/client/scratchpad/test/browser_scratchpad_chrome_context_pref.js
index 7d22814b4036..c58739a6313d 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_chrome_context_pref.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_chrome_context_pref.js
@@ -12,11 +12,11 @@ function test()
   Services.prefs.setBoolPref(DEVTOOLS_CHROME_ENABLED, true);
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,Scratchpad test for bug 646070 - chrome context preference";
+  gBrowser.loadURI("data:text/html,Scratchpad test for bug 646070 - chrome context preference");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_contexts.js b/devtools/client/scratchpad/test/browser_scratchpad_contexts.js
index e41b82500be8..738ece698a0b 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_contexts.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_contexts.js
@@ -7,11 +7,11 @@ function test() {
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,test context switch in Scratchpad";
+  gBrowser.loadURI("data:text/html,test context switch in Scratchpad");
 }
 
 function runTests() {
@@ -49,7 +49,7 @@ function runTests() {
       ok(!pageResult, "no content.foobarBug636725");
     },
     then: function* () {
-      is(content.wrappedJSObject.foobarBug636725, "aloha",
+      is(gBrowser.contentWindowAsCPOW.wrappedJSObject.foobarBug636725, "aloha",
          "content.foobarBug636725 has been set");
     }
   }, {
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_display_non_error_exceptions.js b/devtools/client/scratchpad/test/browser_scratchpad_display_non_error_exceptions.js
index 7f136a8b77c4..e6957446a900 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_display_non_error_exceptions.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_display_non_error_exceptions.js
@@ -8,12 +8,12 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests, {"state":{"text":""}});
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html, test that exceptions are output as " +
-      "comments correctly in Scratchpad";
+  gBrowser.loadURI("data:text/html, test that exceptions are output as " +
+                   "comments correctly in Scratchpad");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_display_outputs_errors.js b/devtools/client/scratchpad/test/browser_scratchpad_display_outputs_errors.js
index 66616dd2f3a8..a96a3b6119e3 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_display_outputs_errors.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_display_outputs_errors.js
@@ -8,12 +8,12 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests, {"state":{"text":""}});
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test that exceptions are output as " +
-      "comments for 'display' and not sent to the console in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test that exceptions are output as " +
+                   "comments for 'display' and not sent to the console in Scratchpad");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_edit_ui_updates.js b/devtools/client/scratchpad/test/browser_scratchpad_edit_ui_updates.js
index b67d63f0708e..35519817d63e 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_edit_ui_updates.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_edit_ui_updates.js
@@ -13,11 +13,11 @@ function test()
   waitForExplicitFinish();
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
   Services.prefs.setBoolPref(DEVTOOLS_CHROME_ENABLED, false);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,test Edit menu updates Scratchpad - bug 699130";
+  gBrowser.loadURI("data:text/html,test Edit menu updates Scratchpad - bug 699130");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_eval_func.js b/devtools/client/scratchpad/test/browser_scratchpad_eval_func.js
index 7cb1de460ac7..5b7d80c80e34 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_eval_func.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_eval_func.js
@@ -7,11 +7,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,test Scratchpad eval function.";
+  gBrowser.loadURI("data:text/html;charset=utf8,test Scratchpad eval function.");
 }
 
 function reportErrorAndQuit(error) {
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_execute_print.js b/devtools/client/scratchpad/test/browser_scratchpad_execute_print.js
index 950bb5fbb0a3..804cea16f234 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_execute_print.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_execute_print.js
@@ -7,11 +7,11 @@ function test() {
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test run() and display() in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test run() and display() in Scratchpad");
 }
 
 function runTests() {
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_falsy.js b/devtools/client/scratchpad/test/browser_scratchpad_falsy.js
index ad5b593a808b..8dc560fc2e6a 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_falsy.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_falsy.js
@@ -8,11 +8,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(testFalsy);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test falsy display() values in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test falsy display() values in Scratchpad");
 }
 
 function testFalsy()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_files.js b/devtools/client/scratchpad/test/browser_scratchpad_files.js
index 1dba4cdf3325..d39f7a6b4d97 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_files.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_files.js
@@ -16,11 +16,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test file open and save in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test file open and save in Scratchpad");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_goto_line_ui.js b/devtools/client/scratchpad/test/browser_scratchpad_goto_line_ui.js
index 6350ecdab185..b29ba6b476d2 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_goto_line_ui.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_goto_line_ui.js
@@ -8,11 +8,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test the 'Jump to line' feature in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test the 'Jump to line' feature in Scratchpad");
 }
 
 function runTests(aWindow, aScratchpad)
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_help_key.js b/devtools/client/scratchpad/test/browser_scratchpad_help_key.js
index 26116dace001..7688d4c5ce29 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_help_key.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_help_key.js
@@ -8,10 +8,10 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  content.location = "data:text/html,Test keybindings for opening Scratchpad MDN Documentation, bug 650760";
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  gBrowser.loadURI("data:text/html,Test keybindings for opening Scratchpad MDN Documentation, bug 650760");
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTest);
-  }, {capture: true, once: true});
+  });
 }
 
 function runTest()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_initialization.js b/devtools/client/scratchpad/test/browser_scratchpad_initialization.js
index 3f94970d0c52..94239005e2d0 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_initialization.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_initialization.js
@@ -10,11 +10,11 @@ function test()
 
   Services.prefs.setBoolPref(DEVTOOLS_CHROME_ENABLED, false);
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,initialization test for Scratchpad";
+  gBrowser.loadURI("data:text/html,initialization test for Scratchpad");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_inspect.js b/devtools/client/scratchpad/test/browser_scratchpad_inspect.js
index 51657c3bd70b..69ed201bc52f 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_inspect.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_inspect.js
@@ -7,11 +7,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,<p>test inspect() in Scratchpad</p>";
+  gBrowser.loadURI("data:text/html;charset=utf8,<p>test inspect() in Scratchpad</p>");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_long_string.js b/devtools/client/scratchpad/test/browser_scratchpad_long_string.js
index 1012543a3980..797484f52d93 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_long_string.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_long_string.js
@@ -7,11 +7,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,<p>test long string in Scratchpad</p>";
+  gBrowser.loadURI("data:text/html;charset=utf8,<p>test long string in Scratchpad</p>");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_modeline.js b/devtools/client/scratchpad/test/browser_scratchpad_modeline.js
index c384979bd352..dc7ff23c7f54 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_modeline.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_modeline.js
@@ -15,11 +15,11 @@ function test() {
 
   Services.prefs.setBoolPref(DEVTOOLS_CHROME_ENABLED, false);
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test file open and save in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test file open and save in Scratchpad");
 }
 
 function runTests() {
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_open_error_console.js b/devtools/client/scratchpad/test/browser_scratchpad_open_error_console.js
index 298091039e92..379a5c1ece3a 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_open_error_console.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_open_error_console.js
@@ -9,12 +9,12 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,test Scratchpad." +
-                     "openErrorConsole()";
+  gBrowser.loadURI("data:text/html;charset=utf8,test Scratchpad." +
+                   "openErrorConsole()");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_pprint-02.js b/devtools/client/scratchpad/test/browser_scratchpad_pprint-02.js
index 996f5711d032..92f50d72387e 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_pprint-02.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_pprint-02.js
@@ -7,11 +7,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,test Scratchpad pretty print.";
+  gBrowser.loadURI("data:text/html;charset=utf8,test Scratchpad pretty print.");
 }
 
 var gTabsize;
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_pprint.js b/devtools/client/scratchpad/test/browser_scratchpad_pprint.js
index 28ff0cd6b0a5..222e76b1cafb 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_pprint.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_pprint.js
@@ -7,11 +7,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,test Scratchpad pretty print.";
+  gBrowser.loadURI("data:text/html;charset=utf8,test Scratchpad pretty print.");
 }
 
 function runTests(sw)
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_pprint_error_goto_line.js b/devtools/client/scratchpad/test/browser_scratchpad_pprint_error_goto_line.js
index 006771133fc1..3b7e74c3ce28 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_pprint_error_goto_line.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_pprint_error_goto_line.js
@@ -10,12 +10,12 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,"
-    + "test Scratchpad pretty print error goto line.";
+  gBrowser.loadURI("data:text/html;charset=utf8,"
+                   + "test Scratchpad pretty print error goto line.");
 }
 
 function testJumpToPrettyPrintError(sp, error, remark) {
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_recent_files.js b/devtools/client/scratchpad/test/browser_scratchpad_recent_files.js
index a7d99029ef97..185ca7abe792 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_recent_files.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_recent_files.js
@@ -336,11 +336,11 @@ function test()
   PreferenceObserver.init();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(startTest);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test recent files in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test recent files in Scratchpad");
 }
 
 function finishTest()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_reload_and_run.js b/devtools/client/scratchpad/test/browser_scratchpad_reload_and_run.js
index 39bbaa2570a0..3a538925ddf4 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_reload_and_run.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_reload_and_run.js
@@ -17,11 +17,11 @@ function test()
   Services.prefs.setBoolPref(DEVTOOLS_CHROME_ENABLED, true);
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,Scratchpad test for bug 740948";
+  gBrowser.loadURI("data:text/html,Scratchpad test for bug 740948");
 }
 
 function runTests()
@@ -54,8 +54,8 @@ function runTests()
   let browser = gBrowser.selectedBrowser;
 
   let deferred = defer();
-  browser.addEventListener("DOMWindowCreated", function () {
-    browser.contentWindow.addEventListener("foo", function () {
+  browser.contentWindowAsCPOW.addEventListener("DOMWindowCreated", function () {
+    browser.contentWindowAsCPOW.addEventListener("foo", function () {
       is(browser.contentWindow.document.body.innerHTML, "Modified text",
         "After reloading, HTML is different.");
 
@@ -64,7 +64,7 @@ function runTests()
     }, {capture: true, once: true});
   }, {capture: true, once: true});
 
-  ok(browser.contentWindow.document.body.innerHTML !== "Modified text",
+  ok(browser.contentWindowAsCPOW.document.body.innerHTML !== "Modified text",
       "Before reloading, HTML is intact.");
   sp.reloadAndRun().then(deferred.promise).then(finish);
 }
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_remember_view_options.js b/devtools/client/scratchpad/test/browser_scratchpad_remember_view_options.js
index b8db9106fcb3..ec56ef79c330 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_remember_view_options.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_remember_view_options.js
@@ -13,12 +13,12 @@ function test()
   // preference value with the expected value.
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<title>Bug 1140839</title>" +
-    "<p>test Scratchpad should remember View options";
+  gBrowser.loadURI("data:text/html,<title>Bug 1140839</title>" +
+                   "<p>test Scratchpad should remember View options");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_reset_undo.js b/devtools/client/scratchpad/test/browser_scratchpad_reset_undo.js
index 1e922a2e4e21..de773d8f2d91 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_reset_undo.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_reset_undo.js
@@ -25,11 +25,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test that undo get's reset after file load in Scratchpad";
+  gBrowser.loadURI("data:text/html,<p>test that undo get's reset after file load in Scratchpad");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_revert_to_saved.js b/devtools/client/scratchpad/test/browser_scratchpad_revert_to_saved.js
index 4cb1a2b0e12c..21480e175145 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_revert_to_saved.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_revert_to_saved.js
@@ -124,10 +124,10 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(startTest);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<p>test reverting to last saved state of" +
-                     " a file </p>";
+  gBrowser.loadURI("data:text/html,<p>test reverting to last saved state of" +
+                   " a file </p>");
 }
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_run_error_goto_line.js b/devtools/client/scratchpad/test/browser_scratchpad_run_error_goto_line.js
index 4e176b45717b..33059172a40b 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_run_error_goto_line.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_run_error_goto_line.js
@@ -10,11 +10,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,test Scratchpad pretty print.";
+  gBrowser.loadURI("data:text/html;charset=utf8,test Scratchpad pretty print.");
 }
 
 function runTests(sw)
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_tab.js b/devtools/client/scratchpad/test/browser_scratchpad_tab.js
index c48f1b4cf60b..96684d2e50b7 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_tab.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_tab.js
@@ -9,13 +9,13 @@ function test() {
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     Services.prefs.setIntPref("devtools.editor.tabsize", 5);
 
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,Scratchpad test for the Tab key, bug 660560";
+  gBrowser.loadURI("data:text/html,Scratchpad test for the Tab key, bug 660560");
 }
 
 function runTests() {
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_tab_switch.js b/devtools/client/scratchpad/test/browser_scratchpad_tab_switch.js
index 1a55fca95061..5eaaeab772a4 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_tab_switch.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_tab_switch.js
@@ -12,16 +12,16 @@ function test()
 
   tab1 = BrowserTestUtils.addTab(gBrowser);
   gBrowser.selectedTab = tab1;
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     tab2 = BrowserTestUtils.addTab(gBrowser);
     gBrowser.selectedTab = tab2;
-    gBrowser.selectedBrowser.addEventListener("load", function () {
+    BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
       openScratchpad(runTests);
-    }, {capture: true, once: true});
-    content.location = "data:text/html,test context switch in Scratchpad tab 2";
-  }, {capture: true, once: true});
+    });
+    gBrowser.loadURI("data:text/html,test context switch in Scratchpad tab 2");
+  });
 
-  content.location = "data:text/html,test context switch in Scratchpad tab 1";
+  gBrowser.loadURI("data:text/html,test context switch in Scratchpad tab 1");
 }
 
 function runTests()
@@ -52,11 +52,11 @@ function runTests()
 
   sp.setText("window.foosbug653108 = 'aloha';");
 
-  ok(!content.wrappedJSObject.foosbug653108,
+  ok(!gBrowser.contentWindowAsCPOW.wrappedJSObject.foosbug653108,
      "no content.foosbug653108");
 
   sp.run().then(function () {
-    is(content.wrappedJSObject.foosbug653108, "aloha",
+    is(gBrowser.contentWindowAsCPOW.wrappedJSObject.foosbug653108, "aloha",
        "content.foosbug653108 has been set");
 
     gBrowser.tabContainer.addEventListener("TabSelect", runTests2, true);
@@ -75,17 +75,16 @@ function runTests2() {
 
     sp.setText("window.foosbug653108 = 'ahoyhoy';");
     sp.run().then(function () {
-      is(content.wrappedJSObject.foosbug653108, "ahoyhoy",
+      is(gBrowser.contentWindowAsCPOW.wrappedJSObject.foosbug653108, "ahoyhoy",
          "content.foosbug653108 has been set 2");
 
-      gBrowser.selectedBrowser.addEventListener("load", runTests3, true);
-      content.location = "data:text/html,test context switch in Scratchpad location 2";
+      BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(runTests3);
+      gBrowser.loadURI("data:text/html,test context switch in Scratchpad location 2");
     });
   });
 }
 
 function runTests3() {
-  gBrowser.selectedBrowser.removeEventListener("load", runTests3, true);
   // Check that the sandbox is not cached.
 
   sp.setText("typeof foosbug653108;");
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_throw_output.js b/devtools/client/scratchpad/test/browser_scratchpad_throw_output.js
index 2e7970754c1f..9bf3b20cc67b 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_throw_output.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_throw_output.js
@@ -8,11 +8,11 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(testThrowOutput);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,<p>Test throw outputs in Scratchpad</p>";
+  gBrowser.loadURI("data:text/html;charset=utf8,<p>Test throw outputs in Scratchpad</p>");
 }
 
 function testThrowOutput()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_ui.js b/devtools/client/scratchpad/test/browser_scratchpad_ui.js
index 5fda567a5554..d5cb5c1e1da4 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_ui.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_ui.js
@@ -7,12 +7,12 @@ function test()
   waitForExplicitFinish();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(runTests);
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html,<title>foobarBug636725</title>" +
-    "<p>test inspect() in Scratchpad";
+  gBrowser.loadURI("data:text/html,<title>foobarBug636725</title>" +
+                   "<p>test inspect() in Scratchpad");
 }
 
 function runTests()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_unsaved.js b/devtools/client/scratchpad/test/browser_scratchpad_unsaved.js
index 9d2ebdf2c39e..713d673dca4e 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_unsaved.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_unsaved.js
@@ -24,7 +24,7 @@ function test()
   testRestoreFromFileUnsaved();
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  content.location = "data:text/html,<p>test star* UI for unsaved file changes";
+  gBrowser.loadURI("data:text/html,<p>test star* UI for unsaved file changes");
 }
 
 function testListeners()
diff --git a/devtools/client/scratchpad/test/browser_scratchpad_wrong_window_focus.js b/devtools/client/scratchpad/test/browser_scratchpad_wrong_window_focus.js
index 0b3429edf290..670644aef16c 100644
--- a/devtools/client/scratchpad/test/browser_scratchpad_wrong_window_focus.js
+++ b/devtools/client/scratchpad/test/browser_scratchpad_wrong_window_focus.js
@@ -26,7 +26,7 @@ function test()
   // is currently active (it should be the older one).
 
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-  gBrowser.selectedBrowser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(function () {
     openScratchpad(function () {
       let sw = gScratchpadWindow;
       let {require} = Cu.import("resource://devtools/shared/Loader.jsm", {});
@@ -41,9 +41,9 @@ function test()
         });
       });
     });
-  }, {capture: true, once: true});
+  });
 
-  content.location = "data:text/html;charset=utf8,<p>test window focus for Scratchpad.";
+  gBrowser.loadURI("data:text/html;charset=utf8,<p>test window focus for Scratchpad.");
 }
 
 function testFocus(sw, hud) {
diff --git a/devtools/client/scratchpad/test/head.js b/devtools/client/scratchpad/test/head.js
index da55825624b2..27994f03c2b2 100644
--- a/devtools/client/scratchpad/test/head.js
+++ b/devtools/client/scratchpad/test/head.js
@@ -93,10 +93,10 @@ function openTabAndScratchpad(aOptions = {})
   return new promise(resolve => {
     gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
     let {selectedBrowser} = gBrowser;
-    selectedBrowser.addEventListener("load", function () {
+    BrowserTestUtils.browserLoaded(selectedBrowser).then(function () {
       openScratchpad((win, sp) => resolve([win, sp]), aOptions);
-    }, {capture: true, once: true});
-    content.location = "data:text/html;charset=utf8," + (aOptions.tabContent || "");
+    });
+    gBrowser.loadURI("data:text/html;charset=utf8," + (aOptions.tabContent || ""));
   });
 }
 
diff --git a/devtools/client/sourceeditor/test/browser_css_autocompletion.js b/devtools/client/sourceeditor/test/browser_css_autocompletion.js
index ddd13cb7553d..b5815de0a224 100644
--- a/devtools/client/sourceeditor/test/browser_css_autocompletion.js
+++ b/devtools/client/sourceeditor/test/browser_css_autocompletion.js
@@ -81,7 +81,7 @@ function test() {
   waitForExplicitFinish();
   addTab(TEST_URI).then(function () {
     /* eslint-disable mozilla/no-cpows-in-tests */
-    doc = content.document;
+    doc = gBrowser.contentDocumentAsCPOW;
     /* eslint-enable mozilla/no-cpows-in-tests */
     runTests();
   });
diff --git a/devtools/client/sourceeditor/test/browser_css_getInfo.js b/devtools/client/sourceeditor/test/browser_css_getInfo.js
index 9028d8fea189..aeaf7966c4ac 100644
--- a/devtools/client/sourceeditor/test/browser_css_getInfo.js
+++ b/devtools/client/sourceeditor/test/browser_css_getInfo.js
@@ -129,7 +129,7 @@ function test() {
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
   BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => {
     /* eslint-disable mozilla/no-cpows-in-tests */
-    doc = content.document;
+    doc = gBrowser.contentDocumentAsCPOW;
     /* eslint-enable mozilla/no-cpows-in-tests */
     runTests();
   });
diff --git a/devtools/client/sourceeditor/test/browser_css_statemachine.js b/devtools/client/sourceeditor/test/browser_css_statemachine.js
index a992380f6a4c..d50b3336c895 100644
--- a/devtools/client/sourceeditor/test/browser_css_statemachine.js
+++ b/devtools/client/sourceeditor/test/browser_css_statemachine.js
@@ -60,7 +60,7 @@ function test() {
   gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, TEST_URI);
   BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(() => {
     /* eslint-disable mozilla/no-cpows-in-tests */
-    doc = content.document;
+    doc = gBrowser.contentDocumentAsCPOW;
     /* eslint-enable mozilla/no-cpows-in-tests */
     runTests();
   });
diff --git a/devtools/client/storage/test/head.js b/devtools/client/storage/test/head.js
index 6ae580be814b..9b774bf6b4f4 100644
--- a/devtools/client/storage/test/head.js
+++ b/devtools/client/storage/test/head.js
@@ -57,7 +57,7 @@ registerCleanupFunction(() => {
  */
 function* openTab(url, options = {}) {
   let tab = yield addTab(url, options);
-  let content = tab.linkedBrowser.contentWindow;
+  let content = tab.linkedBrowser.contentWindowAsCPOW;
 
   gWindow = content.wrappedJSObject;
 
diff --git a/devtools/client/styleeditor/test/head.js b/devtools/client/styleeditor/test/head.js
index 8d5492a72d53..ca5eafee4f33 100644
--- a/devtools/client/styleeditor/test/head.js
+++ b/devtools/client/styleeditor/test/head.js
@@ -46,13 +46,8 @@ var navigateTo = function (url) {
   info(`Navigating to ${url}`);
   let browser = gBrowser.selectedBrowser;
 
-  return new Promise(resolve => {
-    browser.addEventListener("load", function () {
-      resolve();
-    }, {capture: true, once: true});
-
-    browser.loadURI(url);
-  });
+  browser.loadURI(url);
+  return BrowserTestUtils.browserLoaded(browser);
 };
 
 var navigateToAndWaitForStyleSheets = Task.async(function* (url, ui) {
diff --git a/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_context_menu_open_url.js b/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_context_menu_open_url.js
index 5cdb179a5adc..d205d3ca046d 100644
--- a/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_context_menu_open_url.js
+++ b/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_context_menu_open_url.js
@@ -70,9 +70,7 @@ function listenToTabLoad() {
   return new Promise((resolve) => {
     gBrowser.tabContainer.addEventListener("TabOpen", function (evt) {
       let newTab = evt.target;
-      newTab.linkedBrowser.addEventListener("load", function () {
-        resolve(newTab);
-      }, {capture: true, once: true});
+      BrowserTestUtils.browserLoaded(newTab.linkedBrowser).then(() => resolve(newTab));
     }, {capture: true, once: true});
   });
 }
diff --git a/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_multiple_windows_and_tabs.js b/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_multiple_windows_and_tabs.js
index a35ba9d5df99..55129fef11ec 100644
--- a/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_multiple_windows_and_tabs.js
+++ b/devtools/client/webconsole/new-console-output/test/mochitest/browser_webconsole_multiple_windows_and_tabs.js
@@ -35,7 +35,7 @@ add_task(async function () {
   for (let tab of tabs) {
     // Open the console in tab${i}.
     let hud = await openConsole(tab);
-    let tabWindow = hud.target.tab.linkedBrowser.contentWindow;
+    let tabWindow = hud.target.tab.linkedBrowser.contentWindowAsCPOW;
     let message = "message for tab " + tabs.indexOf(tab);
 
     // Log a message in the newly opened console.
diff --git a/devtools/client/webconsole/new-console-output/test/mochitest/head.js b/devtools/client/webconsole/new-console-output/test/mochitest/head.js
index d9fa31e68f72..29261254984a 100644
--- a/devtools/client/webconsole/new-console-output/test/mochitest/head.js
+++ b/devtools/client/webconsole/new-console-output/test/mochitest/head.js
@@ -211,10 +211,8 @@ function hideContextMenu(hud) {
 }
 
 function loadDocument(url, browser = gBrowser.selectedBrowser) {
-  return new Promise(resolve => {
-    browser.addEventListener("load", resolve, {capture: true, once: true});
-    BrowserTestUtils.loadURI(browser, url);
-  });
+  BrowserTestUtils.loadURI(browser, url);
+  return BrowserTestUtils.browserLoaded(browser);
 }
 
 /**
diff --git a/devtools/client/webconsole/test/browser_bug_638949_copy_link_location.js b/devtools/client/webconsole/test/browser_bug_638949_copy_link_location.js
index dba119056dea..c89ee886bddd 100644
--- a/devtools/client/webconsole/test/browser_bug_638949_copy_link_location.js
+++ b/devtools/client/webconsole/test/browser_bug_638949_copy_link_location.js
@@ -26,7 +26,7 @@ add_task(function* () {
   let menu = hud.iframeWindow.document.getElementById("output-contextmenu");
 
   hud.jsterm.clearOutput();
-  content.console.log("bug 638949");
+  gBrowser.contentWindowAsCPOW.console.log("bug 638949");
 
   // Test that the "Copy Link Location" command is disabled for non-network
   // messages.
@@ -54,7 +54,7 @@ add_task(function* () {
 
   hud.jsterm.clearOutput();
   // Reloading will produce network logging
-  content.location.reload();
+  gBrowser.reload();
 
   // Test that the "Copy Link Location" command is enabled and works
   // as expected for any network-related message.
diff --git a/devtools/client/webconsole/test/browser_console.js b/devtools/client/webconsole/test/browser_console.js
index 5bcddbcba6d7..a0d0afcf9fbd 100644
--- a/devtools/client/webconsole/test/browser_console.js
+++ b/devtools/client/webconsole/test/browser_console.js
@@ -62,7 +62,7 @@ function testMessages(hud) {
   Cu.nukeSandbox(sandbox);
 
   // Add a message from a content window.
-  content.console.log("bug587757b");
+  gBrowser.contentWindowAsCPOW.console.log("bug587757b");
 
   // Test eval.
   hud.jsterm.execute("document.location.href");
diff --git a/devtools/client/webconsole/test/browser_console_certificate_imminent_distrust.js b/devtools/client/webconsole/test/browser_console_certificate_imminent_distrust.js
index e777395ad12b..084010d76e20 100644
--- a/devtools/client/webconsole/test/browser_console_certificate_imminent_distrust.js
+++ b/devtools/client/webconsole/test/browser_console_certificate_imminent_distrust.js
@@ -93,7 +93,7 @@ function runTestLoop(theHud) {
     gHud = theHud;
   }
   gHud.jsterm.clearOutput();
-  gContentBrowser.addEventListener("load", onLoad, true);
+  BrowserTestUtils.browserLoaded(gContentBrowser).then(onLoad);
   if (gCurrentTest.pref) {
     SpecialPowers.pushPrefEnv({"set": gCurrentTest.pref},
       function () {
@@ -105,8 +105,6 @@ function runTestLoop(theHud) {
 }
 
 function onLoad() {
-  gContentBrowser.removeEventListener("load", onLoad, true);
-
   waitForSuccess({
     name: gCurrentTest.name,
     validator: function () {
diff --git a/devtools/client/webconsole/test/browser_console_copy_command.js b/devtools/client/webconsole/test/browser_console_copy_command.js
index 4f434127df31..0c4bdd55127f 100644
--- a/devtools/client/webconsole/test/browser_console_copy_command.js
+++ b/devtools/client/webconsole/test/browser_console_copy_command.js
@@ -46,7 +46,7 @@ add_task(function* testCopy() {
                   [obj.toSource(), JSON.stringify(obj, null, "  ")],
     [
       "$('#" + ID + "')",
-      content.document.getElementById(ID).outerHTML
+      gBrowser.contentDocumentAsCPOW.getElementById(ID).outerHTML
     ]
   ];
   for (let [source, reference] of samples) {
diff --git a/devtools/client/webconsole/test/browser_console_variables_view.js b/devtools/client/webconsole/test/browser_console_variables_view.js
index 811e7de5ff67..b66bdb4e79c4 100644
--- a/devtools/client/webconsole/test/browser_console_variables_view.js
+++ b/devtools/client/webconsole/test/browser_console_variables_view.js
@@ -126,7 +126,7 @@ function onTestPropFound(aResults) {
 
 function onFooObjFetchAfterUpdate(aVar) {
   info("onFooObjFetchAfterUpdate");
-  let expectedValue = content.document.title + content.location +
+  let expectedValue = gBrowser.contentTitle + gBrowser.currentURI.spec +
                       "[object HTMLParagraphElement]";
 
   return findVariableViewProperties(aVar, [
@@ -138,7 +138,7 @@ function onUpdatedTestPropFound(aResults) {
   let prop = aResults[0].matchedProp;
   ok(prop, "matched the updated |testProp| property value");
 
-  is(content.wrappedJSObject.fooObj.testProp, aResults[0].value,
+  is(gBrowser.contentWindowAsCPOW.wrappedJSObject.fooObj.testProp, aResults[0].value,
      "|fooObj.testProp| value has been updated");
 
   // Check that property name updates work.
@@ -168,9 +168,9 @@ function onRenamedTestPropFound(aResults) {
   let prop = aResults[0].matchedProp;
   ok(prop, "matched the renamed |testProp| property");
 
-  ok(!content.wrappedJSObject.fooObj.testProp,
+  ok(!gBrowser.contentWindowAsCPOW.wrappedJSObject.fooObj.testProp,
      "|fooObj.testProp| has been deleted");
-  is(content.wrappedJSObject.fooObj.testUpdatedProp, aResults[0].value,
+  is(gBrowser.contentWindowAsCPOW.wrappedJSObject.fooObj.testUpdatedProp, aResults[0].value,
      "|fooObj.testUpdatedProp| is correct");
 
   // Check that property value updates that cause exceptions are reported in
@@ -223,6 +223,6 @@ function testPropDelete(aProp) {
   return waitForSuccess({
     name: "property deleted",
     timeout: 60000,
-    validator: () => !("testUpdatedProp" in content.wrappedJSObject.fooObj)
+    validator: () => !("testUpdatedProp" in gBrowser.contentWindowAsCPOW.wrappedJSObject.fooObj)
   });
 }
diff --git a/devtools/client/webconsole/test/browser_netmonitor_shows_reqs_in_webconsole.js b/devtools/client/webconsole/test/browser_netmonitor_shows_reqs_in_webconsole.js
index c54bbc02b3cb..13d5a5c4d1e7 100644
--- a/devtools/client/webconsole/test/browser_netmonitor_shows_reqs_in_webconsole.js
+++ b/devtools/client/webconsole/test/browser_netmonitor_shows_reqs_in_webconsole.js
@@ -55,9 +55,9 @@ add_task(function* () {
 function loadDocument(browser) {
   let deferred = defer();
 
-  browser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(browser).then(function () {
     deferred.resolve();
-  }, {capture: true, once: true});
+  });
   BrowserTestUtils.loadURI(gBrowser.selectedBrowser, TEST_PATH);
 
   return deferred.promise;
diff --git a/devtools/client/webconsole/test/browser_output_breaks_after_console_dir_uninspectable.js b/devtools/client/webconsole/test/browser_output_breaks_after_console_dir_uninspectable.js
index 38a5b54198ec..092661a6e7a9 100644
--- a/devtools/client/webconsole/test/browser_output_breaks_after_console_dir_uninspectable.js
+++ b/devtools/client/webconsole/test/browser_output_breaks_after_console_dir_uninspectable.js
@@ -34,7 +34,7 @@ add_task(function* () {
     }],
   });
 
-  content.console.log("fooBug773466b");
+  gBrowser.contentWindowAsCPOW.console.log("fooBug773466b");
 
   yield waitForMessages({
     webconsole: hud,
diff --git a/devtools/client/webconsole/test/browser_webconsole_bug_580030_errors_after_page_reload.js b/devtools/client/webconsole/test/browser_webconsole_bug_580030_errors_after_page_reload.js
index af00bf913a38..6a45e0282257 100644
--- a/devtools/client/webconsole/test/browser_webconsole_bug_580030_errors_after_page_reload.js
+++ b/devtools/client/webconsole/test/browser_webconsole_bug_580030_errors_after_page_reload.js
@@ -21,13 +21,13 @@ function test() {
     executeSoon(() => {
       hud.jsterm.clearOutput();
       info("wait for reload");
-      content.location.reload();
+      gBrowser.reload();
     });
 
     yield hud.target.once("navigate");
     info("target navigated");
 
-    let button = content.document.querySelector("button");
+    let button = gBrowser.contentDocumentAsCPOW.querySelector("button");
     ok(button, "button found");
 
     // On e10s, the exception is triggered in child process
@@ -36,7 +36,7 @@ function test() {
       expectUncaughtException();
     }
 
-    EventUtils.sendMouseEvent({type: "click"}, button, content);
+    EventUtils.sendMouseEvent({type: "click"}, button, gBrowser.contentWindowAsCPOW);
 
     yield waitForMessages({
       webconsole: hud,
diff --git a/devtools/client/webconsole/test/browser_webconsole_bug_597103_deactivateHUDForContext_unfocused_window.js b/devtools/client/webconsole/test/browser_webconsole_bug_597103_deactivateHUDForContext_unfocused_window.js
index e14c3a069d41..32ac8b7eebff 100644
--- a/devtools/client/webconsole/test/browser_webconsole_bug_597103_deactivateHUDForContext_unfocused_window.js
+++ b/devtools/client/webconsole/test/browser_webconsole_bug_597103_deactivateHUDForContext_unfocused_window.js
@@ -19,12 +19,10 @@ function tab1Loaded() {
 function win2Loaded() {
   tab2 = win2.gBrowser.addTab(TEST_URI);
   win2.gBrowser.selectedTab = tab2;
-  tab2.linkedBrowser.addEventListener("load", tab2Loaded, true);
+  BrowserTestUtils.browserLoaded(tab2.linkedBrowser).then(tab2Loaded);
 }
 
 function tab2Loaded(aEvent) {
-  tab2.linkedBrowser.removeEventListener(aEvent.type, tab2Loaded, true);
-
   let consolesOpened = 0;
   function onWebConsoleOpen() {
     consolesOpened++;
diff --git a/devtools/client/webconsole/test/browser_webconsole_bug_597460_filter_scroll.js b/devtools/client/webconsole/test/browser_webconsole_bug_597460_filter_scroll.js
index 8b1fd88159f4..28390bac04fc 100644
--- a/devtools/client/webconsole/test/browser_webconsole_bug_597460_filter_scroll.js
+++ b/devtools/client/webconsole/test/browser_webconsole_bug_597460_filter_scroll.js
@@ -26,7 +26,7 @@ function consoleOpened(hud) {
   let deferred = defer();
 
   for (let i = 0; i < 200; i++) {
-    content.console.log("test message " + i);
+    gBrowser.contentWindowAsCPOW.console.log("test message " + i);
   }
 
   hud.setFilterState("network", false);
@@ -53,7 +53,7 @@ function consoleOpened(hud) {
       }],
     }).then(deferred.resolve);
 
-    content.location.reload();
+    gBrowser.reload();
   });
 
   return deferred.promise;
diff --git a/devtools/client/webconsole/test/browser_webconsole_bug_632275_getters_document_width.js b/devtools/client/webconsole/test/browser_webconsole_bug_632275_getters_document_width.js
index 45d1f7102d61..78b288dd8386 100644
--- a/devtools/client/webconsole/test/browser_webconsole_bug_632275_getters_document_width.js
+++ b/devtools/client/webconsole/test/browser_webconsole_bug_632275_getters_document_width.js
@@ -17,7 +17,7 @@ function test() {
 }
 
 function consoleOpened(hud) {
-  let doc = content.wrappedJSObject.document;
+  let doc = gBrowser.contentWindowAsCPOW.wrappedJSObject.document;
   getterValue = doc.foobar._val;
   hud.jsterm.execute("console.dir(document)");
 
@@ -26,7 +26,7 @@ function consoleOpened(hud) {
 }
 
 function onViewOpened(hud, event, view) {
-  let doc = content.wrappedJSObject.document;
+  let doc = gBrowser.contentWindowAsCPOW.wrappedJSObject.document;
 
   findVariableViewProperties(view, [
     { name: /^(width|height)$/, dontMatch: 1 },
diff --git a/devtools/client/webconsole/test/browser_webconsole_bug_644419_log_limits.js b/devtools/client/webconsole/test/browser_webconsole_bug_644419_log_limits.js
index 93063e4364f6..4a52a316980e 100644
--- a/devtools/client/webconsole/test/browser_webconsole_bug_644419_log_limits.js
+++ b/devtools/client/webconsole/test/browser_webconsole_bug_644419_log_limits.js
@@ -66,7 +66,7 @@ function testWebDevLimits() {
 function testWebDevLimits2() {
   // Fill the log with Web Developer errors.
   for (let i = 0; i < 11; i++) {
-    content.console.log("test message " + i);
+    gBrowser.contentWindowAsCPOW.console.log("test message " + i);
   }
 
   return waitForMessages({
@@ -91,7 +91,7 @@ function testJsLimits() {
   Services.prefs.setIntPref("devtools.hud.loglimit.exception", 10);
 
   hud.jsterm.clearOutput();
-  content.console.log("testing JS limits");
+  gBrowser.contentWindowAsCPOW.console.log("testing JS limits");
 
   // Find the sentinel entry.
   return waitForMessages({
@@ -106,9 +106,9 @@ function testJsLimits() {
 
 function testJsLimits2() {
   // Fill the log with JS errors.
-  let head = content.document.getElementsByTagName("head")[0];
+  let head = gBrowser.contentDocumentAsCPOW.getElementsByTagName("head")[0];
   for (let i = 0; i < 11; i++) {
-    let script = content.document.createElement("script");
+    let script = gBrowser.contentDocumentAsCPOW.createElement("script");
     script.text = "fubar" + i + ".bogus(6);";
 
     if (!Services.appinfo.browserTabsRemoteAutostart) {
@@ -141,7 +141,7 @@ function testNetLimits() {
   Services.prefs.setIntPref("devtools.hud.loglimit.network", 10);
 
   hud.jsterm.clearOutput();
-  content.console.log("testing Net limits");
+  gBrowser.contentWindowAsCPOW.console.log("testing Net limits");
 
   // Find the sentinel entry.
   return waitForMessages({
@@ -159,9 +159,9 @@ function testNetLimits() {
 
 function loadImage() {
   if (gCounter < 11) {
-    let body = content.document.getElementsByTagName("body")[0];
+    let body = gBrowser.contentDocumentAsCPOW.getElementsByTagName("body")[0];
     gImage && gImage.removeEventListener("load", loadImage, true);
-    gImage = content.document.createElement("img");
+    gImage = gBrowser.contentDocumentAsCPOW.createElement("img");
     gImage.src = "test-image.png?_fubar=" + gCounter;
     body.insertBefore(gImage, body.firstChild);
     gImage.addEventListener("load", loadImage, true);
@@ -194,7 +194,7 @@ function testCssLimits() {
   Services.prefs.setIntPref("devtools.hud.loglimit.cssparser", 10);
 
   hud.jsterm.clearOutput();
-  content.console.log("testing CSS limits");
+  gBrowser.contentWindowAsCPOW.console.log("testing CSS limits");
 
   // Find the sentinel entry.
   return waitForMessages({
@@ -209,9 +209,9 @@ function testCssLimits() {
 
 function testCssLimits2() {
   // Fill the log with CSS errors.
-  let body = content.document.getElementsByTagName("body")[0];
+  let body = gBrowser.contentDocumentAsCPOW.getElementsByTagName("body")[0];
   for (let i = 0; i < 11; i++) {
-    let div = content.document.createElement("div");
+    let div = gBrowser.contentDocumentAsCPOW.createElement("div");
     div.setAttribute("style", "-moz-foobar" + i + ": 42;");
     body.insertBefore(div, body.firstChild);
   }
diff --git a/devtools/client/webconsole/test/browser_webconsole_console_extras.js b/devtools/client/webconsole/test/browser_webconsole_console_extras.js
index 078e33119644..eeffc4754c5f 100644
--- a/devtools/client/webconsole/test/browser_webconsole_console_extras.js
+++ b/devtools/client/webconsole/test/browser_webconsole_console_extras.js
@@ -37,7 +37,7 @@ function consoleOpened(hud) {
     finishTest();
   });
 
-  let button = content.document.querySelector("button");
+  let button = gBrowser.contentDocumentAsCPOW.querySelector("button");
   ok(button, "we have the button");
-  EventUtils.sendMouseEvent({ type: "click" }, button, content);
+  EventUtils.sendMouseEvent({ type: "click" }, button, gBrowser.contentWindowAsCPOW);
 }
diff --git a/devtools/client/webconsole/test/browser_webconsole_console_logging_api.js b/devtools/client/webconsole/test/browser_webconsole_console_logging_api.js
index 317337543cd2..3e7a4e10810c 100644
--- a/devtools/client/webconsole/test/browser_webconsole_console_logging_api.js
+++ b/devtools/client/webconsole/test/browser_webconsole_console_logging_api.js
@@ -24,7 +24,7 @@ add_task(function* () {
 });
 
 function* testMethod(method, hud, outputNode) {
-  let console = content.console;
+  let console = gBrowser.contentWindowAsCPOW.console;
 
   hud.jsterm.clearOutput();
 
diff --git a/devtools/client/webconsole/test/browser_webconsole_log_file_filter.js b/devtools/client/webconsole/test/browser_webconsole_log_file_filter.js
index 7e722a054b36..4b8ca43d2316 100644
--- a/devtools/client/webconsole/test/browser_webconsole_log_file_filter.js
+++ b/devtools/client/webconsole/test/browser_webconsole_log_file_filter.js
@@ -25,7 +25,7 @@ add_task(function* () {
 });
 
 function consoleOpened() {
-  let console = content.console;
+  let console = gBrowser.contentWindowAsCPOW.console;
   console.log("sentinel log");
   return waitForMessages({
     webconsole: hud,
@@ -63,7 +63,7 @@ function countMessageNodes() {
   let outputNode = hud.outputNode;
 
   let messageNodes = outputNode.querySelectorAll(".message");
-  content.console.log(messageNodes.length);
+  gBrowser.contentWindowAsCPOW.console.log(messageNodes.length);
   let displayedMessageNodes = 0;
   let view = hud.iframeWindow;
   for (let i = 0; i < messageNodes.length; i++) {
diff --git a/devtools/client/webconsole/test/browser_webconsole_notifications.js b/devtools/client/webconsole/test/browser_webconsole_notifications.js
index 1cac2dcdcae2..3fb6fbc1dbfd 100644
--- a/devtools/client/webconsole/test/browser_webconsole_notifications.js
+++ b/devtools/client/webconsole/test/browser_webconsole_notifications.js
@@ -26,7 +26,7 @@ function waitForEvents(onConsoleOpened) {
   function webConsoleCreated(id) {
     Services.obs.removeObserver(observer, "web-console-created");
     ok(HUDService.getHudReferenceById(id), "We have a hud reference");
-    content.wrappedJSObject.console.log("adding a log message");
+    gBrowser.contentWindowAsCPOW.wrappedJSObject.console.log("adding a log message");
   }
 
   function webConsoleDestroyed(id) {
diff --git a/devtools/client/webconsole/test/browser_webconsole_output_copy_newlines.js b/devtools/client/webconsole/test/browser_webconsole_output_copy_newlines.js
index 991b765f7322..d2321cec82ab 100644
--- a/devtools/client/webconsole/test/browser_webconsole_output_copy_newlines.js
+++ b/devtools/client/webconsole/test/browser_webconsole_output_copy_newlines.js
@@ -19,8 +19,8 @@ add_task(function* () {
                    .getControllerForCommand("cmd_copy");
   is(controller.isCommandEnabled("cmd_copy"), false, "cmd_copy is disabled");
 
-  content.console.log("Hello world! bug916997a");
-  content.console.log("Hello world 2! bug916997b");
+  gBrowser.contentWindowAsCPOW.console.log("Hello world! bug916997a");
+  gBrowser.contentWindowAsCPOW.console.log("Hello world 2! bug916997b");
 
   yield waitForMessages({
     webconsole: hud,
diff --git a/devtools/client/webconsole/test/browser_webconsole_reflow.js b/devtools/client/webconsole/test/browser_webconsole_reflow.js
index 86caa10e050f..bdb270ad0a43 100644
--- a/devtools/client/webconsole/test/browser_webconsole_reflow.js
+++ b/devtools/client/webconsole/test/browser_webconsole_reflow.js
@@ -14,8 +14,8 @@ add_task(function* () {
   let hud = yield openConsole();
 
   function onReflowListenersReady() {
-    browser.contentDocument.body.style.display = "none";
-    browser.contentDocument.body.clientTop;
+    browser.contentDocumentAsCPOW.body.style.display = "none";
+    browser.contentDocumentAsCPOW.body.clientTop;
   }
 
   Services.prefs.setBoolPref("devtools.webconsole.filter.csslog", true);
diff --git a/devtools/client/webconsole/test/browser_webconsole_shows_reqs_in_netmonitor.js b/devtools/client/webconsole/test/browser_webconsole_shows_reqs_in_netmonitor.js
index 740dfd39445d..9d21ccda835a 100644
--- a/devtools/client/webconsole/test/browser_webconsole_shows_reqs_in_netmonitor.js
+++ b/devtools/client/webconsole/test/browser_webconsole_shows_reqs_in_netmonitor.js
@@ -52,9 +52,9 @@ add_task(function* () {
 function loadDocument(browser) {
   let deferred = defer();
 
-  browser.addEventListener("load", function () {
+  BrowserTestUtils.browserLoaded(browser).then(function () {
     deferred.resolve();
-  }, {capture: true, once: true});
+  });
   BrowserTestUtils.loadURI(gBrowser.selectedBrowser, TEST_PATH);
 
   return deferred.promise;
diff --git a/devtools/client/webconsole/test/head.js b/devtools/client/webconsole/test/head.js
index ab1fa4acc24d..b23c80557b8d 100644
--- a/devtools/client/webconsole/test/head.js
+++ b/devtools/client/webconsole/test/head.js
@@ -227,7 +227,7 @@ var waitForTab = Task.async(function* () {
   yield once(gBrowser.tabContainer, "TabOpen");
   let tab = gBrowser.selectedTab;
   let browser = tab.linkedBrowser;
-  yield once(browser, "load", true);
+  yield BrowserTestUtils.browserLoaded(browser);
   info("The tab load completed");
   return tab;
 });
diff --git a/devtools/server/tests/browser/browser_markers-docloading-01.js b/devtools/server/tests/browser/browser_markers-docloading-01.js
index efb86c23aef6..fa33852bf4a9 100644
--- a/devtools/server/tests/browser/browser_markers-docloading-01.js
+++ b/devtools/server/tests/browser/browser_markers-docloading-01.js
@@ -12,7 +12,7 @@ const MARKER_NAMES = ["document::DOMContentLoaded", "document::Load"];
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  let doc = browser.contentDocument;
+  let doc = browser.contentDocumentAsCPOW;
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_markers-docloading-02.js b/devtools/server/tests/browser/browser_markers-docloading-02.js
index 14c60fc1e21d..11b6e01d1762 100644
--- a/devtools/server/tests/browser/browser_markers-docloading-02.js
+++ b/devtools/server/tests/browser/browser_markers-docloading-02.js
@@ -12,7 +12,7 @@ const MARKER_NAMES = ["document::DOMContentLoaded", "document::Load"];
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  let doc = browser.contentDocument;
+  let doc = browser.contentDocumentAsCPOW;
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_markers-docloading-03.js b/devtools/server/tests/browser/browser_markers-docloading-03.js
index 2f31bcc99343..d73e25cc2436 100644
--- a/devtools/server/tests/browser/browser_markers-docloading-03.js
+++ b/devtools/server/tests/browser/browser_markers-docloading-03.js
@@ -12,7 +12,7 @@ const MARKER_NAMES = ["document::DOMContentLoaded", "document::Load"];
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "doc_innerHTML.html");
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  let doc = browser.contentDocument;
+  let doc = browser.contentDocumentAsCPOW;
 
   initDebuggerServer();
   let client = new DebuggerClient(DebuggerServer.connectPipe());
diff --git a/devtools/server/tests/browser/browser_storage_dynamic_windows.js b/devtools/server/tests/browser/browser_storage_dynamic_windows.js
index 71ab2adb7697..690f1febe614 100644
--- a/devtools/server/tests/browser/browser_storage_dynamic_windows.js
+++ b/devtools/server/tests/browser/browser_storage_dynamic_windows.js
@@ -164,10 +164,10 @@ function testAddIframe(front) {
     front.on("stores-update", onStoresUpdate);
 
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    let iframe = content.document.createElement("iframe");
+    let iframe = gBrowser.contentDocumentAsCPOW.createElement("iframe");
     iframe.src = ALT_DOMAIN_SECURED + "storage-secured-iframe.html";
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    content.document.querySelector("body").appendChild(iframe);
+    gBrowser.contentDocumentAsCPOW.querySelector("body").appendChild(iframe);
   });
 }
 
diff --git a/devtools/server/tests/browser/browser_storage_updates.js b/devtools/server/tests/browser/browser_storage_updates.js
index 8e6cc9587d3e..c160b8e7bfe5 100644
--- a/devtools/server/tests/browser/browser_storage_updates.js
+++ b/devtools/server/tests/browser/browser_storage_updates.js
@@ -299,7 +299,7 @@ function* finishTests(client) {
 add_task(function* () {
   let browser = yield addTab(MAIN_DOMAIN + "storage-updates.html");
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  let doc = browser.contentDocument;
+  let doc = browser.contentDocumentAsCPOW;
 
   initDebuggerServer();
 
diff --git a/docshell/test/browser/browser_bug420605.js b/docshell/test/browser/browser_bug420605.js
index 7dcbcc2531ea..16c39b2eeba1 100644
--- a/docshell/test/browser/browser_bug420605.js
+++ b/docshell/test/browser/browser_bug420605.js
@@ -103,8 +103,6 @@ function test() {
     historyService.addObserver(historyObserver);
 
     function onPageLoad() {
-      gBrowser.selectedBrowser
-              .removeEventListener("DOMContentLoaded", arguments.callee, true);
       clickLinkIfReady();
     }
 
@@ -116,7 +114,6 @@ function test() {
 
     // Now open the test page in a new tab.
     gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser);
-    gBrowser.selectedBrowser.addEventListener(
-        "DOMContentLoaded", onPageLoad, true);
-    content.location = pageurl;
+    BrowserTestUtils.waitForContentEvent(gBrowser.selectedBrowser, "DOMContentLoaded", true).then(onPageLoad);
+    gBrowser.selectedBrowser.loadURI(pageurl);
 }
diff --git a/docshell/test/browser/browser_bug673467.js b/docshell/test/browser/browser_bug673467.js
index 6f746c896b28..a7ffd68fdf3e 100644
--- a/docshell/test/browser/browser_bug673467.js
+++ b/docshell/test/browser/browser_bug673467.js
@@ -19,32 +19,35 @@ var doc = "data:text/html,<html><body onload='load()'>" +
 function test() {
   waitForExplicitFinish();
 
-  let tab = BrowserTestUtils.addTab(gBrowser, doc);
-  let tabBrowser = tab.linkedBrowser;
+  let taskFinished;
 
-  BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => {
-    return ContentTask.spawn(tab.linkedBrowser, null, () => {
+  let tab = BrowserTestUtils.addTab(gBrowser, doc, {}, (tab) => {
+    taskFinished = ContentTask.spawn(tab.linkedBrowser, null, () => {
       return new Promise(resolve => {
-        // The main page has loaded.  Now wait for the iframe to load.
-        let iframe = content.document.getElementById('iframe');
-        iframe.addEventListener('load', function listener(aEvent) {
+        addEventListener("load", function() {
+          // The main page has loaded.  Now wait for the iframe to load.
+          let iframe = content.document.getElementById('iframe');
+          iframe.addEventListener('load', function listener(aEvent) {
 
-          // Wait for the iframe to load the new document, not about:blank.
-          if (!iframe.src)
-            return;
+            // Wait for the iframe to load the new document, not about:blank.
+            if (!iframe.src)
+              return;
 
-          iframe.removeEventListener('load', listener, true);
-          let shistory = content
-                          .QueryInterface(Ci.nsIInterfaceRequestor)
-                          .getInterface(Ci.nsIWebNavigation)
-                          .sessionHistory;
+            iframe.removeEventListener('load', listener, true);
+            let shistory = content
+                            .QueryInterface(Ci.nsIInterfaceRequestor)
+                            .getInterface(Ci.nsIWebNavigation)
+                            .sessionHistory;
 
-          Assert.equal(shistory.count, 1, "shistory count should be 1.");
-          resolve();
+            Assert.equal(shistory.count, 1, "shistory count should be 1.");
+            resolve();
+          }, true);
         }, true);
       });
     });
-  }).then(() => {
+  });
+
+  taskFinished.then(() => {
     gBrowser.removeTab(tab);
     finish();
   });
diff --git a/docshell/test/browser/browser_history_triggeringprincipal_viewsource.js b/docshell/test/browser/browser_history_triggeringprincipal_viewsource.js
index 453c696ad7bc..39f9557a48ae 100644
--- a/docshell/test/browser/browser_history_triggeringprincipal_viewsource.js
+++ b/docshell/test/browser/browser_history_triggeringprincipal_viewsource.js
@@ -37,7 +37,7 @@ add_task(async function() {
     popupPromise = BrowserTestUtils.waitForEvent(backCtxtMenu, "popupshown");
     BrowserTestUtils.synthesizeMouseAtCenter("body", { type: "contextmenu", button: 2 }, aBrowser);
     await popupPromise;
-    loadPromise = BrowserTestUtils.browserLoaded(tab.linkedBrowser, false, VIEW_SRC_URI);
+    loadPromise = BrowserTestUtils.waitForContentEvent(tab.linkedBrowser, "pageshow");
     let backItem = backCtxtMenu.getElementsByAttribute("id", "context-back")[0];
     backItem.click();
     backCtxtMenu.hidePopup();
diff --git a/docshell/test/browser/browser_loadDisallowInherit.js b/docshell/test/browser/browser_loadDisallowInherit.js
index 8fd0a9776129..69188dbc79e7 100644
--- a/docshell/test/browser/browser_loadDisallowInherit.js
+++ b/docshell/test/browser/browser_loadDisallowInherit.js
@@ -19,12 +19,9 @@ function startTest() {
   let browser = gBrowser.getBrowserForTab(tab);
 
   function loadURL(url, flags, func) {
-    browser.addEventListener("load", function loadListener(e) {
-      if (browser.currentURI.spec != url)
-        return;
-      browser.removeEventListener(e.type, loadListener, true);
+    BrowserTestUtils.browserLoaded(browser, false, url).then(() => {
       func();
-    }, true);
+    });
     browser.loadURIWithFlags(url, flags, null, null, null);
   }
 
diff --git a/docshell/test/browser/head.js b/docshell/test/browser/head.js
index c9d21277f95b..ac3e59c34c34 100644
--- a/docshell/test/browser/head.js
+++ b/docshell/test/browser/head.js
@@ -51,9 +51,7 @@ function timelineTestOpenUrl(url) {
     let tab = window.gBrowser.selectedTab = window.gBrowser.addTab(url);
     let linkedBrowser = tab.linkedBrowser;
 
-    linkedBrowser.addEventListener("load", function() {
-      resolve(tab);
-    }, {capture: true, once: true});
+    BrowserTestUtils.browserLoaded(linkedBrowser).then(() => resolve(tab));
   });
 
   return Promise.all([tabSwitchPromise, loadPromise]).then(([_, tab]) => tab);
diff --git a/docshell/test/navigation/browser_bug343515.js b/docshell/test/navigation/browser_bug343515.js
index 4fb0779e79d5..cf4b1d7a7ff7 100644
--- a/docshell/test/navigation/browser_bug343515.js
+++ b/docshell/test/navigation/browser_bug343515.js
@@ -9,11 +9,12 @@ var ctx = {};
 // how many load events should happen on that page (one for the toplevel doc
 // and one for each subframe) and wait until we receive the expected number
 // of events.
-function nShotsListener(aElem, aType, aCallback, aCount) {
+function nShotsListener(aBrowser, aType, aCallback, aCount) {
   let count = aCount;
-  aElem.addEventListener(aType, function listenerCallback() {
+  let removeFunc;
+  removeFunc = BrowserTestUtils.addContentEventListener(aBrowser, aType, function listenerCallback() {
     if (--count == 0) {
-      aElem.removeEventListener(aType, listenerCallback, true);
+      removeFunc();
 
       // aCallback is executed asynchronously, which is handy because load
       // events fire before mIsDocumentLoaded is actually set to true. :(
@@ -22,8 +23,8 @@ function nShotsListener(aElem, aType, aCallback, aCount) {
   }, true);
 }
 
-function oneShotListener(aElem, aType, aCallback) {
-  nShotsListener(aElem, aType, aCallback, 1);
+function oneShotListener(aBrowser, aType, aCallback) {
+  nShotsListener(aBrowser, aType, aCallback, 1);
 }
 
 function waitForPageshow(aBrowser, callback) {
diff --git a/dom/base/test/browser_aboutnewtab_process_selection.js b/dom/base/test/browser_aboutnewtab_process_selection.js
index 2872f9746e5f..43dc14c22da0 100644
--- a/dom/base/test/browser_aboutnewtab_process_selection.js
+++ b/dom/base/test/browser_aboutnewtab_process_selection.js
@@ -17,7 +17,8 @@ async function ensurePreloaded(gBrowser) {
   // We cannot use the regular BrowserTestUtils helper for waiting here, since that
   // would try to insert the preloaded browser, which would only break things.
   await BrowserTestUtils.waitForCondition( () => {
-    return gBrowser._preloadedBrowser.contentDocument.readyState == "complete";
+    let doc = gBrowser._preloadedBrowser.contentDocumentAsCPOW;
+    return doc && doc.readyState == "complete";
   });
 }
 
diff --git a/dom/base/test/browser_bug1058164.js b/dom/base/test/browser_bug1058164.js
index 472b4ac3f1ce..8ddc7e227efb 100644
--- a/dom/base/test/browser_bug1058164.js
+++ b/dom/base/test/browser_bug1058164.js
@@ -25,13 +25,15 @@ function prepareForVisibilityEvents(browser, expectedOrder) {
   return new Promise((resolve) => {
     let order = [];
 
+    let rmvHide, rmvShow;
+
     let checkSatisfied = () => {
       if (order.length < expectedOrder.length) {
         // We're still waiting...
         return;
       } else {
-        browser.removeEventListener("pagehide", eventListener);
-        browser.removeEventListener("pageshow", eventListener);
+        rmvHide();
+        rmvShow();
 
         for (let i = 0; i < expectedOrder.length; ++i) {
           is(order[i], expectedOrder[i], "Got expected event");
@@ -40,15 +42,19 @@ function prepareForVisibilityEvents(browser, expectedOrder) {
       }
     };
 
-    let eventListener = (e) => {
-      if (e.persisted) {
-        order.push(e.type);
-        checkSatisfied();
-      }
+    let eventListener = (type) => {
+      order.push(type);
+      checkSatisfied();
     };
 
-    browser.addEventListener("pagehide", eventListener);
-    browser.addEventListener("pageshow", eventListener);
+    let checkFn = (e) => e.persisted;
+
+    rmvHide = BrowserTestUtils.addContentEventListener(browser, "pagehide",
+                                                       () => eventListener("pagehide"),
+                                                       false, checkFn, false, false);
+    rmvShow = BrowserTestUtils.addContentEventListener(browser, "pageshow",
+                                                       () => eventListener("pageshow"),
+                                                       false, checkFn, false, false);
   });
 }
 
diff --git a/dom/html/test/browser_bug592641.js b/dom/html/test/browser_bug592641.js
index e83987777d27..f29e79bab91c 100644
--- a/dom/html/test/browser_bug592641.js
+++ b/dom/html/test/browser_bug592641.js
@@ -11,7 +11,7 @@ function test() {
 
   ctx.tab1 = BrowserTestUtils.addTab(gBrowser, testPath + "bug592641_img.jpg");
   ctx.tab1Browser = gBrowser.getBrowserForTab(ctx.tab1);
-  ctx.tab1Browser.addEventListener("load", load1Soon, true);
+  BrowserTestUtils.browserLoaded(ctx.tab1Browser).then(load1Soon);
 }
 
 function checkTitle(title) {
@@ -22,7 +22,6 @@ function checkTitle(title) {
 }
 
 function load1Soon() {
-  ctx.tab1Browser.removeEventListener("load", load1Soon, true);
   // onload is fired in OnStopDecode, so let's use executeSoon() to make sure
   // that any other OnStopDecode event handlers get the chance to fire first.
   executeSoon(load1Done);
@@ -37,11 +36,10 @@ function load1Done() {
   // the cached case.
   ctx.tab2 = BrowserTestUtils.addTab(gBrowser, testPath + "bug592641_img.jpg");
   ctx.tab2Browser = gBrowser.getBrowserForTab(ctx.tab2);
-  ctx.tab2Browser.addEventListener("load", load2Soon, true);
+  BrowserTestUtils.browserLoaded(ctx.tab2Browser).then(load2Soon);
 }
 
 function load2Soon() {
-  ctx.tab2Browser.removeEventListener("load", load2Soon, true);
   // onload is fired in OnStopDecode, so let's use executeSoon() to make sure
   // that any other OnStopDecode event handlers get the chance to fire first.
   executeSoon(load2Done);
diff --git a/dom/security/test/csp/browser_test_web_manifest.js b/dom/security/test/csp/browser_test_web_manifest.js
index da650c4470b4..364614cba5d7 100644
--- a/dom/security/test/csp/browser_test_web_manifest.js
+++ b/dom/security/test/csp/browser_test_web_manifest.js
@@ -191,7 +191,7 @@ add_task(async function() {
 });
 
 async function testObtainingManifest(aBrowser, aTest) {
-  const waitForObserver = waitForNetObserver(aTest);
+  const waitForObserver = waitForNetObserver(aBrowser, aTest);
   // Expect an exception (from promise rejection) if there a content policy
   // that is violated.
   try {
@@ -206,19 +206,19 @@ async function testObtainingManifest(aBrowser, aTest) {
 }
 
 // Helper object used to observe policy violations when blocking is expected.
-function waitForNetObserver(aTest) {
-  return new Promise((resolve) => {
-    // We don't need to wait for violation, so just resolve
-    if (!aTest.expected.includes("block")){
-      return resolve();
-    }
-    const observer = {
-      observe(subject, topic) {
-        SpecialPowers.removeObserver(observer, "csp-on-violate-policy");
-        aTest.run(topic);
+function waitForNetObserver(aBrowser, aTest) {
+  // We don't need to wait for violation, so just resolve
+  if (!aTest.expected.includes("block")){
+    return Promise.resolve();
+  }
+
+  return ContentTask.spawn(aBrowser, null, () => {
+    return new Promise(resolve => {
+      function observe(subject, topic) {
+        Services.obs.removeObserver(observe, "csp-on-violate-policy");
         resolve();
-      },
-    };
-    SpecialPowers.addObserver(observer, "csp-on-violate-policy");
-  });
+      };
+      Services.obs.addObserver(observe, "csp-on-violate-policy");
+    });
+  }).then(() => aTest.run("csp-on-violate-policy"));
 }
diff --git a/dom/tests/browser/browser_ConsoleStoragePBTest_perwindowpb.js b/dom/tests/browser/browser_ConsoleStoragePBTest_perwindowpb.js
index e3874e66d965..a9ed18b8116c 100644
--- a/dom/tests/browser/browser_ConsoleStoragePBTest_perwindowpb.js
+++ b/dom/tests/browser/browser_ConsoleStoragePBTest_perwindowpb.js
@@ -29,7 +29,7 @@ function test() {
   }
 
   function doTest(aIsPrivateMode, aWindow, aCallback) {
-    aWindow.gBrowser.selectedBrowser.addEventListener("load", function() {
+    BrowserTestUtils.browserLoaded(aWindow.gBrowser.selectedBrowser).then(() => {
       consoleObserver = {
         observe: function(aSubject, aTopic, aData) {
           if (aTopic == "console-api-log-event") {
@@ -48,7 +48,7 @@ function test() {
       aWindow.Services.obs.addObserver(
         consoleObserver, "console-api-log-event");
       aWindow.nativeConsole.log("foo bar baz (private: " + aIsPrivateMode + ")");
-    }, {capture: true, once: true});
+    });
 
     // We expect that console API messages are always stored.
     storageShouldOccur = true;
diff --git a/dom/url/tests/browser_download_after_revoke.js b/dom/url/tests/browser_download_after_revoke.js
index a1aeb4cea3f5..0cf0c7a85e68 100644
--- a/dom/url/tests/browser_download_after_revoke.js
+++ b/dom/url/tests/browser_download_after_revoke.js
@@ -4,7 +4,6 @@ function test () {
 
   function onLoad() {
     info("Page loaded.");
-    gBrowser.selectedBrowser.removeEventListener("load", onLoad, true);
 
     var listener = {
       onOpenWindow: function(aXULWindow) {
@@ -46,8 +45,8 @@ function test () {
     });
   }
 
-  gBrowser.selectedBrowser.addEventListener("load", onLoad, true);
+  BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser).then(onLoad);
 
   info("Loading download page...");
-  content.location = "http://example.com/browser/dom/url/tests/empty.html";
+  gBrowser.loadURI("http://example.com/browser/dom/url/tests/empty.html");
 }
diff --git a/dom/workers/test/head.js b/dom/workers/test/head.js
index c598f55cbf63..a3b7c203da92 100644
--- a/dom/workers/test/head.js
+++ b/dom/workers/test/head.js
@@ -15,11 +15,7 @@ function addTab(url) {
   gBrowser.selectedTab = tab;
   let linkedBrowser = tab.linkedBrowser;
   linkedBrowser.messageManager.loadFrameScript(FRAME_SCRIPT_URL, false);
-  return new Promise(function (resolve) {
-    linkedBrowser.addEventListener("load", function() {
-      resolve(tab);
-    }, {capture: true, once: true});
-  });
+  return BrowserTestUtils.browserLoaded(linkedBrowser).then(() => tab);
 }
 
 /**
diff --git a/security/manager/ssl/tests/mochitest/browser/browser_bug627234_perwindowpb.js b/security/manager/ssl/tests/mochitest/browser/browser_bug627234_perwindowpb.js
index 35923e1da2b7..c1beac572cda 100644
--- a/security/manager/ssl/tests/mochitest/browser/browser_bug627234_perwindowpb.js
+++ b/security/manager/ssl/tests/mochitest/browser/browser_bug627234_perwindowpb.js
@@ -49,7 +49,7 @@ function test() {
   }
 
   function doTest(aIsPrivateMode, aWindow, aCallback) {
-    aWindow.gBrowser.selectedBrowser.addEventListener("load", function() {
+    BrowserTestUtils.browserLoaded(aWindow.gBrowser.selectedBrowser).then(() => {
       let sslStatus = new FakeSSLStatus();
       uri = aWindow.Services.io.newURI("https://localhost/img.png");
       gSSService.processHeader(Ci.nsISiteSecurityService.HEADER_HSTS, uri,
@@ -60,7 +60,7 @@ function test() {
                                 "checking sts host");
 
       aCallback();
-    }, {capture: true, once: true});
+    });
 
     aWindow.gBrowser.selectedBrowser.loadURI(testURI);
   }
diff --git a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
index 3a8f3760193c..b0d9e335736d 100644
--- a/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
+++ b/testing/mochitest/BrowserTestUtils/BrowserTestUtils.jsm
@@ -1581,11 +1581,27 @@ this.BrowserTestUtils = {
    * Opens a tab with a given uri and params object. If the params object is not set
    * or the params parameter does not include a triggeringPricnipal then this function
    * provides a params object using the systemPrincipal as the default triggeringPrincipal.
+   *
+   * @param {xul:tabbrowser} tabbrowser
+   *        The gBrowser object to open the tab with.
+   * @param {string} uri
+   *        The URI to open in the new tab.
+   * @param {object} params [optional]
+   *        Parameters object for gBrowser.addTab.
+   * @param {function} beforeLoadFunc [optional]
+   *        A function to run after that xul:browser has been created but before the URL is
+   *        loaded. Can spawn a content task in the tab, for example.
    */
-  addTab(browser, uri, params = {}) {
+  addTab(tabbrowser, uri, params = {}, beforeLoadFunc = null) {
     if (!params.triggeringPrincipal) {
       params.triggeringPrincipal = Services.scriptSecurityManager.getSystemPrincipal();
     }
-    return browser.addTab(uri, params);
+    if (beforeLoadFunc) {
+      let window = tabbrowser.ownerGlobal;
+      window.addEventListener("TabOpen", function(e) {
+        beforeLoadFunc(e.target);
+      }, {once: true});
+    }
+    return tabbrowser.addTab(uri, params);
   }
 };
diff --git a/testing/mochitest/tests/browser/browser_waitForFocus.js b/testing/mochitest/tests/browser/browser_waitForFocus.js
index 81852ec4a3a5..2665da74e34e 100644
--- a/testing/mochitest/tests/browser/browser_waitForFocus.js
+++ b/testing/mochitest/tests/browser/browser_waitForFocus.js
@@ -3,22 +3,11 @@ const gBaseURL = "https://example.com/browser/testing/mochitest/tests/browser/";
 
 function promiseTabLoadEvent(tab, url)
 {
-  return new Promise(function (resolve, reject) {
-    function handleLoadEvent(event) {
-      if (event.originalTarget != tab.linkedBrowser.contentDocument ||
-          event.target.location.href == "about:blank" ||
-          (url && event.target.location.href != url)) {
-        return;
-      }
-
-      tab.linkedBrowser.removeEventListener("load", handleLoadEvent, true);
-      resolve(event);
-    }
-
-    tab.linkedBrowser.addEventListener("load", handleLoadEvent, true, true);
-    if (url)
-      tab.linkedBrowser.loadURI(url);
-  });
+  let promise = BrowserTestUtils.browserLoaded(tab.linkedBrowser, false, url);
+  if (url) {
+    tab.linkedBrowser.loadURI(url);
+  }
+  return promise;
 }
 
 // Load a new blank tab
diff --git a/toolkit/components/passwordmgr/test/browser/browser_passwordmgr_switchtab.js b/toolkit/components/passwordmgr/test/browser/browser_passwordmgr_switchtab.js
index c9814c46ee33..5ad2f2fc0a60 100644
--- a/toolkit/components/passwordmgr/test/browser/browser_passwordmgr_switchtab.js
+++ b/toolkit/components/passwordmgr/test/browser/browser_passwordmgr_switchtab.js
@@ -35,9 +35,7 @@ add_task(async function test() {
     gBrowser.removeTab(tab);
   });
 
-  tab.linkedBrowser.addEventListener("load", () => {
-    finish();
-  }, true);
+  BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(() => finish());
   tab.linkedBrowser.loadURI("http://example.com/browser/toolkit/components/passwordmgr/test/browser/authenticate.sjs");
 
   });
diff --git a/toolkit/components/payments/test/browser/browser_request_summary.js b/toolkit/components/payments/test/browser/browser_request_summary.js
index 71232e09220a..9cdbfffbf4aa 100644
--- a/toolkit/components/payments/test/browser/browser_request_summary.js
+++ b/toolkit/components/payments/test/browser/browser_request_summary.js
@@ -6,6 +6,6 @@ add_task(async function test_summary() {
     url: "resource://payments/paymentRequest.xhtml",
   }, async browser => {
     // eslint-disable-next-line mozilla/no-cpows-in-tests
-    ok(browser.contentDocument.getElementById("cancel"), "Cancel button exists");
+    ok(browser.contentDocumentAsCPOW.getElementById("cancel"), "Cancel button exists");
   });
 });
diff --git a/toolkit/components/places/tests/browser/browser_favicon_privatebrowsing_perwindowpb.js b/toolkit/components/places/tests/browser/browser_favicon_privatebrowsing_perwindowpb.js
index 0c1aefc98778..392087b1ceea 100644
--- a/toolkit/components/places/tests/browser/browser_favicon_privatebrowsing_perwindowpb.js
+++ b/toolkit/components/places/tests/browser/browser_favicon_privatebrowsing_perwindowpb.js
@@ -23,9 +23,7 @@ function test() {
   }
 
   function waitForTabLoad(aWin, aCallback) {
-    aWin.gBrowser.selectedBrowser.addEventListener("load", function() {
-      aCallback();
-    }, {capture: true, once: true});
+    BrowserTestUtils.browserLoaded(aWin.gBrowser.selectedBrowser).then(aCallback);
     aWin.gBrowser.selectedBrowser.loadURI(pageURI);
   }
 
diff --git a/toolkit/components/reader/test/browser_readerMode_with_anchor.js b/toolkit/components/reader/test/browser_readerMode_with_anchor.js
index 80afff8eea99..b5c466b1b7b9 100644
--- a/toolkit/components/reader/test/browser_readerMode_with_anchor.js
+++ b/toolkit/components/reader/test/browser_readerMode_with_anchor.js
@@ -29,7 +29,8 @@ add_task(async function() {
     let readerButton = document.getElementById("reader-mode-button");
     readerButton.click();
     await pageShownPromise;
-    is(content.document.documentElement.scrollTop, 0, "scrollTop should be 0");
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    is(gBrowser.contentDocumentAsCPOW.documentElement.scrollTop, 0, "scrollTop should be 0");
     await BrowserTestUtils.synthesizeMouseAtCenter("#foo-anchor", {}, browser);
     await ContentTask.spawn(browser, null, async function() {
       let foo = content.document.getElementById("foo");
diff --git a/toolkit/components/thumbnails/test/browser_thumbnails_bug726727.js b/toolkit/components/thumbnails/test/browser_thumbnails_bug726727.js
index cbff8dce8c3c..292aa0ec27ee 100644
--- a/toolkit/components/thumbnails/test/browser_thumbnails_bug726727.js
+++ b/toolkit/components/thumbnails/test/browser_thumbnails_bug726727.js
@@ -9,10 +9,12 @@ function* runTests() {
   // Create a tab that shows an error page.
   let tab = BrowserTestUtils.addTab(gBrowser, "http://127.0.0.1:1/");
   let browser = tab.linkedBrowser;
-  yield browser.addEventListener("DOMContentLoaded", function() {
+  yield BrowserTestUtils.waitForContentEvent(browser, "DOMContentLoaded");
+
+  yield new Promise(resolve => {
     PageThumbs.shouldStoreThumbnail(browser, (aResult) => {
       ok(!aResult, "we're not going to capture an error page");
-      executeSoon(next);
+      resolve();
     });
-  }, {once: true});
+  });
 }
diff --git a/toolkit/components/thumbnails/test/browser_thumbnails_privacy.js b/toolkit/components/thumbnails/test/browser_thumbnails_privacy.js
index 00a3c168fcca..26973321882a 100644
--- a/toolkit/components/thumbnails/test/browser_thumbnails_privacy.js
+++ b/toolkit/components/thumbnails/test/browser_thumbnails_privacy.js
@@ -62,7 +62,7 @@ function testCombination(combi, url, aCombinations, aResult) {
   let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, url);
   let browser = gBrowser.selectedBrowser;
 
-  whenLoaded(browser, () => {
+  BrowserTestUtils.browserLoaded(browser).then(() => {
     let msg = JSON.stringify(combi) + " == " + aResult;
     PageThumbs.shouldStoreThumbnail(browser, (aIsSafeSite) => {
       is(aIsSafeSite, aResult, msg);
diff --git a/toolkit/components/thumbnails/test/head.js b/toolkit/components/thumbnails/test/head.js
index bf6c5667b46b..c704625c709f 100644
--- a/toolkit/components/thumbnails/test/head.js
+++ b/toolkit/components/thumbnails/test/head.js
@@ -96,7 +96,8 @@ function next(aValue) {
  */
 function addTab(aURI, aCallback) {
   let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, aURI);
-  whenLoaded(tab.linkedBrowser, aCallback);
+  let callback = aCallback ? aCallback : next;
+  BrowserTestUtils.browserLoaded(tab.linkedBrowser).then(callback);
 }
 
 /**
@@ -105,7 +106,7 @@ function addTab(aURI, aCallback) {
  */
 function navigateTo(aURI) {
   let browser = gBrowser.selectedBrowser;
-  whenLoaded(browser);
+  BrowserTestUtils.browserLoaded(browser).then(next);
   browser.loadURI(aURI);
 }
 
diff --git a/toolkit/content/tests/browser/browser_autoplay_policy_play_twice.js b/toolkit/content/tests/browser/browser_autoplay_policy_play_twice.js
index addff873b2a5..a70aae827c30 100644
--- a/toolkit/content/tests/browser/browser_autoplay_policy_play_twice.js
+++ b/toolkit/content/tests/browser/browser_autoplay_policy_play_twice.js
@@ -14,7 +14,7 @@ async function allow_play_for_played_video() {
   let tab = await BrowserTestUtils.openNewForegroundTab(window.gBrowser,
                                                         "about:blank");
   info("- create video -");
-  let document = tab.linkedBrowser.contentDocument;
+  let document = tab.linkedBrowser.contentDocumentAsCPOW;
   let video = document.createElement("video");
   video.src = FILE;
   video.controls = true;
diff --git a/toolkit/content/tests/browser/browser_autoplay_policy_user_gestures.js b/toolkit/content/tests/browser/browser_autoplay_policy_user_gestures.js
index 0bd68458c0e9..333ae07c1b97 100644
--- a/toolkit/content/tests/browser/browser_autoplay_policy_user_gestures.js
+++ b/toolkit/content/tests/browser/browser_autoplay_policy_user_gestures.js
@@ -20,7 +20,8 @@ function setup_test_preference() {
 }
 
 function simulateUserGesture(gesture, targetBrowser) {
-  let targetElement = targetBrowser.contentDocument.documentElement;
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
+  let targetElement = targetBrowser.contentDocumentAsCPOW.documentElement;
   info(`- simulate ${gesture.type} event -`);
   switch (gesture.type) {
     case UserGestures.MOUSE_CLICK:
@@ -42,7 +43,8 @@ async function test_play_without_user_gesture() {
   let tab = await BrowserTestUtils.openNewForegroundTab(window.gBrowser,
                                                         "about:blank");
   info("- create autoplay video -");
-  let document = tab.linkedBrowser.contentDocument;
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
+  let document = tab.linkedBrowser.contentDocumentAsCPOW;
   let video = document.createElement("video");
   video.src = FILE;
   video.autoplay = true;
@@ -67,7 +69,8 @@ async function test_play_with_user_gesture(gesture) {
   let tab = await BrowserTestUtils.openNewForegroundTab(window.gBrowser,
                                                         "about:blank");
   info("- create autoplay video -");
-  let document = tab.linkedBrowser.contentDocument;
+  // eslint-disable-next-line mozilla/no-cpows-in-tests
+  let document = tab.linkedBrowser.contentDocumentAsCPOW;
   let video = document.createElement("video");
   video.src = FILE;
   document.body.appendChild(video);
diff --git a/toolkit/content/tests/browser/browser_contentTitle.js b/toolkit/content/tests/browser/browser_contentTitle.js
index c9c56dafa167..118cfc395a48 100644
--- a/toolkit/content/tests/browser/browser_contentTitle.js
+++ b/toolkit/content/tests/browser/browser_contentTitle.js
@@ -2,12 +2,7 @@ var url = "https://example.com/browser/toolkit/content/tests/browser/file_conten
 
 add_task(async function() {
   let tab = gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, url);
-  await new Promise((resolve) => {
-    addEventListener("TestLocationChange", function listener() {
-      removeEventListener("TestLocationChange", listener);
-      resolve();
-    }, true, true);
-  });
+  await BrowserTestUtils.waitForContentEvent(tab.linkedBrowser, "TestLocationChange", true, null, true);
 
   is(gBrowser.contentTitle, "Test Page", "Should have the right title.");
 
diff --git a/toolkit/content/tests/browser/browser_datetime_datepicker.js b/toolkit/content/tests/browser/browser_datetime_datepicker.js
index df6e5a87b8ec..1104e3902f66 100644
--- a/toolkit/content/tests/browser/browser_datetime_datepicker.js
+++ b/toolkit/content/tests/browser/browser_datetime_datepicker.js
@@ -168,7 +168,7 @@ add_task(async function test_datepicker_clicked() {
   });
 
   // eslint-disable-next-line mozilla/no-cpows-in-tests
-  Assert.equal(content.document.querySelector("input").value, firstDayOnCalendar);
+  Assert.equal(gBrowser.contentDocumentAsCPOW.querySelector("input").value, firstDayOnCalendar);
 
   await helper.tearDown();
 });
diff --git a/toolkit/content/tests/browser/browser_saveImageURL.js b/toolkit/content/tests/browser/browser_saveImageURL.js
index 6b16e91d384e..970aaea43ca9 100644
--- a/toolkit/content/tests/browser/browser_saveImageURL.js
+++ b/toolkit/content/tests/browser/browser_saveImageURL.js
@@ -37,7 +37,8 @@ add_task(async function preferred_API() {
     });
 
     saveImageURL(url, "image.jpg", null, true, false, null, null, null, null, false);
-    let channel = content.document.docShell.currentDocumentChannel;
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    let channel = gBrowser.contentDocumentAsCPOW.docShell.currentDocumentChannel;
     if (channel) {
       ok(true, channel.QueryInterface(Ci.nsIHttpChannelInternal)
                       .channelIsForDownload);
@@ -73,7 +74,8 @@ add_task(async function deprecated_API() {
     // pass the XUL document instead to test this interface.
     let doc = document;
 
-    let channel = content.document.docShell.currentDocumentChannel;
+    // eslint-disable-next-line mozilla/no-cpows-in-tests
+    let channel = gBrowser.contentDocumentAsCPOW.docShell.currentDocumentChannel;
     if (channel) {
       ok(true, channel.QueryInterface(Ci.nsIHttpChannelInternal)
                       .channelIsForDownload);
diff --git a/toolkit/modules/tests/browser/browser_RemotePageManager.js b/toolkit/modules/tests/browser/browser_RemotePageManager.js
index 3e004acb7a26..51d0060d26c0 100644
--- a/toolkit/modules/tests/browser/browser_RemotePageManager.js
+++ b/toolkit/modules/tests/browser/browser_RemotePageManager.js
@@ -80,14 +80,8 @@ add_task(async function init_navigate() {
   let port = await waitForPort(TEST_URL);
   is(port.browser, gBrowser.selectedBrowser, "Port is for the correct browser");
 
-  let loaded = new Promise(resolve => {
-    function listener() {
-      gBrowser.selectedBrowser.removeEventListener("load", listener, true);
-      resolve();
-    }
-    gBrowser.selectedBrowser.addEventListener("load", listener, true);
-    gBrowser.loadURI("about:blank");
-  });
+  let loaded = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+  gBrowser.loadURI("about:blank");
 
   await waitForMessage(port, "RemotePage:Unload");
 
diff --git a/toolkit/modules/tests/browser/browser_WebRequest_cookies.js b/toolkit/modules/tests/browser/browser_WebRequest_cookies.js
index b1aaa6507d8a..ad0b8daa735d 100644
--- a/toolkit/modules/tests/browser/browser_WebRequest_cookies.js
+++ b/toolkit/modules/tests/browser/browser_WebRequest_cookies.js
@@ -78,9 +78,5 @@ add_task(async function filter_urls() {
 });
 
 function waitForLoad(browser = gBrowser.selectedBrowser) {
-  return new Promise(resolve => {
-    browser.addEventListener("load", function() {
-      resolve();
-    }, {capture: true, once: true});
-  });
+  return BrowserTestUtils.browserLoaded(browser);
 }
diff --git a/toolkit/modules/tests/browser/browser_WebRequest_filtering.js b/toolkit/modules/tests/browser/browser_WebRequest_filtering.js
index 7d9f34c1d90f..f6dc344283b5 100644
--- a/toolkit/modules/tests/browser/browser_WebRequest_filtering.js
+++ b/toolkit/modules/tests/browser/browser_WebRequest_filtering.js
@@ -108,9 +108,5 @@ add_task(async function filter_types() {
 });
 
 function waitForLoad(browser = gBrowser.selectedBrowser) {
-  return new Promise(resolve => {
-    browser.addEventListener("load", function() {
-      resolve();
-    }, {capture: true, once: true});
-  });
+  return BrowserTestUtils.browserLoaded(browser);
 }
diff --git a/toolkit/mozapps/extensions/test/browser/browser_bug562797.js b/toolkit/mozapps/extensions/test/browser/browser_bug562797.js
index baff77792699..2002c587e2d9 100644
--- a/toolkit/mozapps/extensions/test/browser/browser_bug562797.js
+++ b/toolkit/mozapps/extensions/test/browser/browser_bug562797.js
@@ -388,6 +388,20 @@ add_test(function() {
   });
 });
 
+function wait_for_page_show(browser) {
+  let promise = new Promise(resolve => {
+    let removeFunc;
+    let listener = () => {
+      removeFunc();
+      resolve();
+    };
+    removeFunc = BrowserTestUtils.addContentEventListener(browser, "pageshow", listener, false,
+                                                          (event) => event.target.location == "http://example.com/",
+                                                          false, false);
+  });
+  return promise;
+}
+
 // Tests than navigating to a website and then going back returns to the
 // previous view
 add_test(function() {
@@ -397,10 +411,7 @@ add_test(function() {
     is_in_list(aManager, "addons://list/plugin", false, false);
 
     gBrowser.loadURI("http://example.com/");
-    gBrowser.addEventListener("pageshow", function listener(event) {
-      if (event.target.location != "http://example.com/")
-        return;
-      gBrowser.removeEventListener("pageshow", listener);
+    wait_for_page_show(gBrowser.selectedBrowser).then(() => {
       info("Part 2");
 
       executeSoon(function() {
@@ -419,10 +430,7 @@ add_test(function() {
             is_in_list(aManager, "addons://list/plugin", false, true);
 
             executeSoon(() => go_forward());
-            gBrowser.addEventListener("pageshow", function listener(event) {
-              if (event.target.location != "http://example.com/")
-                return;
-              gBrowser.removeEventListener("pageshow", listener);
+            wait_for_page_show(gBrowser.selectedBrowser).then(() => {
               info("Part 4");
 
               executeSoon(function() {
@@ -540,11 +548,7 @@ add_test(function() {
         is_in_detail(aManager, "addons://search/", true, false);
 
         gBrowser.loadURI("http://example.com/");
-        gBrowser.addEventListener("pageshow", function listener(event) {
-          if (event.target.location != "http://example.com/")
-            return;
-          gBrowser.removeEventListener("pageshow", listener);
-
+        wait_for_page_show(gBrowser.selectedBrowser).then(() => {
           info("Part 4");
           executeSoon(function() {
             ok(gBrowser.canGoBack, "Should be able to go back");