diff --git a/testing/specialpowers/content/SpecialPowersObserverAPI.js b/testing/specialpowers/content/SpecialPowersObserverAPI.js
index a88e46acc57a..33b7db875d1f 100644
--- a/testing/specialpowers/content/SpecialPowersObserverAPI.js
+++ b/testing/specialpowers/content/SpecialPowersObserverAPI.js
@@ -208,14 +208,8 @@ SpecialPowersObserverAPI.prototype = {
     input.close();
 
     var status;
-    try {
-      channel.QueryInterface(Ci.nsIHttpChannel);
+    if (channel instanceof Ci.nsIHttpChannel) {
       status = channel.responseStatus;
-    } catch(e) {
-      /* The channel is not a nsIHttpCHannel, but that's fine */
-      dump("-*- _readUrlAsString: Got an error while fetching " +
-           "chrome script '" + aUrl + "': (" + e.name + ") " + e.message + ". " +
-           "Ignoring.\n");
     }
 
     if (status == 404) {
diff --git a/toolkit/components/extensions/test/mochitest/chrome.ini b/toolkit/components/extensions/test/mochitest/chrome.ini
index d9ca44b6cb10..206572ad217f 100644
--- a/toolkit/components/extensions/test/mochitest/chrome.ini
+++ b/toolkit/components/extensions/test/mochitest/chrome.ini
@@ -1,6 +1,7 @@
 [DEFAULT]
 support-files =
   chrome_head.js
+  chrome_cleanup_script.js
   head.js
   head_cookies.js
   file_sample.html
diff --git a/toolkit/components/extensions/test/mochitest/chrome_cleanup_script.js b/toolkit/components/extensions/test/mochitest/chrome_cleanup_script.js
new file mode 100644
index 000000000000..d0ae08d00df2
--- /dev/null
+++ b/toolkit/components/extensions/test/mochitest/chrome_cleanup_script.js
@@ -0,0 +1,47 @@
+"use strict";
+
+/* global addMessageListener, sendAsyncMessage */
+
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+function* iterBrowserWindows() {
+  let enm = Services.wm.getEnumerator("navigator:browser");
+  while (enm.hasMoreElements()) {
+    let win = enm.getNext();
+    if (!win.closed && win.gBrowser) {
+      yield win;
+    }
+  }
+}
+
+let initialTabs = new Map();
+for (let win of iterBrowserWindows()) {
+  initialTabs.set(win, new Set(win.gBrowser.tabs));
+}
+
+addMessageListener("check-cleanup", extensionId => {
+  let results = {
+    extraWindows: [],
+    extraTabs: [],
+  };
+
+  for (let win of iterBrowserWindows()) {
+    if (initialTabs.has(win)) {
+      let tabs = initialTabs.get(win);
+
+      for (let tab of win.gBrowser.tabs) {
+        if (!tabs.has(tab)) {
+          results.extraTabs.push(tab.linkedBrowser.currentURI.spec);
+        }
+      }
+    } else {
+      results.extraWindows.push(
+        Array.from(win.gBrowser.tabs,
+                   tab => tab.linkedBrowser.currentURI.spec));
+    }
+  }
+
+  initialTabs = null;
+
+  sendAsyncMessage("cleanup-results", results);
+});
diff --git a/toolkit/components/extensions/test/mochitest/head.js b/toolkit/components/extensions/test/mochitest/head.js
index 1bcffb50f3ff..509d9848a5fd 100644
--- a/toolkit/components/extensions/test/mochitest/head.js
+++ b/toolkit/components/extensions/test/mochitest/head.js
@@ -1,5 +1,7 @@
 "use strict";
 
+let {AppConstants} = SpecialPowers.Cu.import("resource://gre/modules/AppConstants.jsm", {});
+
 // We run tests under two different configurations, from mochitest.ini and
 // mochitest-remote.ini. When running from mochitest-remote.ini, the tests are
 // copied to the sub-directory "test-oop-extensions", which we detect here, and
@@ -14,6 +16,22 @@ if (location.pathname.includes("test-oop-extensions")) {
   SpecialPowers.setIntPref("dom.ipc.keepProcessesAlive.extension", 1);
 }
 
+if (AppConstants.MOZ_BUILD_APP === "browser") {
+  let chromeScript = SpecialPowers.loadChromeScript(
+    SimpleTest.getTestFileURL("chrome_cleanup_script.js"));
+
+  SimpleTest.registerCleanupFunction(async () => {
+    chromeScript.sendAsyncMessage("check-cleanup");
+
+    let results = await chromeScript.promiseOneMessage("cleanup-results");
+    chromeScript.destroy();
+
+    if (results.extraWindows.length || results.extraTabs.length) {
+      ok(false, `Test left extra windows or tabs: ${JSON.stringify(results)}\n`);
+    }
+  });
+}
+
 /* exported waitForLoad */
 
 function waitForLoad(win) {
diff --git a/toolkit/components/extensions/test/mochitest/mochitest-common.ini b/toolkit/components/extensions/test/mochitest/mochitest-common.ini
index 7188f93e63c3..c1468228a361 100644
--- a/toolkit/components/extensions/test/mochitest/mochitest-common.ini
+++ b/toolkit/components/extensions/test/mochitest/mochitest-common.ini
@@ -1,5 +1,6 @@
 [DEFAULT]
 support-files =
+  chrome_cleanup_script.js
   head.js
   file_mixed.html
   head_webrequest.js
diff --git a/toolkit/components/extensions/test/mochitest/test_ext_webrequest_filter.html b/toolkit/components/extensions/test/mochitest/test_ext_webrequest_filter.html
index 62e8159ad109..32319bacd69c 100644
--- a/toolkit/components/extensions/test/mochitest/test_ext_webrequest_filter.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_webrequest_filter.html
@@ -172,6 +172,8 @@ add_task(function* test_webRequest_filter_background() {
 
 add_task(function* teardown() {
   testWindow.close();
+
+  yield new Promise(resolve => setTimeout(resolve, 0));
 });
 </script>
 </head>