diff --git a/testing/marionette/assert.js b/testing/marionette/assert.js
index 0cfba6f4cce2..c7e13e2c0c21 100644
--- a/testing/marionette/assert.js
+++ b/testing/marionette/assert.js
@@ -287,25 +287,6 @@ assert.callable = function(obj, msg = "") {
   return assert.that(o => typeof o == "function", msg)(obj);
 };
 
-/**
- * Asserts that <var>obj</var> is an unsigned short number.
- *
- * @param {?} obj
- *     Value to test.
- * @param {string=} msg
- *     Custom error message.
- *
- * @return {number}
- *     <var>obj</var> is returned unaltered.
- *
- * @throws {InvalidArgumentError}
- *     If <var>obj</var> is not an unsigned short.
- */
-assert.unsignedShort = function(obj, msg = "") {
-  msg = msg || pprint`Expected ${obj} to be >= 0 and < 65536`;
-  return assert.that(n => n >= 0 && n < 65536, msg)(obj);
-};
-
 /**
  * Asserts that <var>obj</var> is an integer.
  *
diff --git a/testing/marionette/browser.js b/testing/marionette/browser.js
index 068bf702049b..8d30646baf7e 100644
--- a/testing/marionette/browser.js
+++ b/testing/marionette/browser.js
@@ -567,17 +567,17 @@ browser.Context = class {
 };
 
 /**
- * The window storage is used to save browsing context ids mapped to weak
+ * The window storage is used to save outer window IDs mapped to weak
  * references of Window objects.
  *
  * Usage:
  *
  *     let wins = new browser.Windows();
- *     wins.set(browser.browsingContext.id, window);
+ *     wins.set(browser.outerWindowID, window);
  *
  *     ...
  *
- *     let win = wins.get(browser.browsingContext.id);
+ *     let win = wins.get(browser.outerWindowID);
  *
  */
 browser.Windows = class extends Map {
@@ -585,7 +585,7 @@ browser.Windows = class extends Map {
    * Save a weak reference to the Window object.
    *
    * @param {string} id
-   *     Browsing context id.
+   *     Outer window ID.
    * @param {Window} win
    *     Window object to save.
    *
@@ -602,7 +602,7 @@ browser.Windows = class extends Map {
    * Get the window object stored by provided |id|.
    *
    * @param {string} id
-   *     Browsing context id.
+   *     Outer window ID.
    *
    * @return {Window}
    *     Saved window object.
diff --git a/testing/marionette/driver.js b/testing/marionette/driver.js
index c3789818aefb..f381bb51b7e0 100644
--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -288,7 +288,7 @@ Object.defineProperty(GeckoDriver.prototype, "chromeWindowHandles", {
     let hs = [];
 
     for (let win of this.windows) {
-      hs.push(getWindowId(win));
+      hs.push(getOuterWindowId(win));
     }
 
     return hs;
@@ -474,16 +474,16 @@ GeckoDriver.prototype.addFrameCloseListener = function(action) {
  * @return {string}
  *     Returns the unique server-assigned ID of the window.
  */
-GeckoDriver.prototype.addBrowser = function(win) {
-  let context = new browser.Context(win, this);
-  let winId = getWindowId(win);
+GeckoDriver.prototype.addBrowser = function(window) {
+  let bc = new browser.Context(window, this);
+  let winId = getOuterWindowId(window);
 
-  this.browsers[winId] = context;
+  this.browsers[winId] = bc;
   this.curBrowser = this.browsers[winId];
   if (!this.wins.has(winId)) {
     // add this to seenItems so we can guarantee
     // the user will get winId as this window's id
-    this.wins.set(winId, win);
+    this.wins.set(winId, window);
   }
 };
 
@@ -579,6 +579,8 @@ GeckoDriver.prototype.getVisibleText = function(el, lines) {
  * their type they are either accepted or ignored.
  */
 GeckoDriver.prototype.registerBrowser = function(id, be) {
+  let listenerWindow = Services.wm.getOuterWindowWithId(id);
+
   // We want to ignore frames that are XUL browsers that aren't in the "main"
   // tabbrowser, but accept things on Fennec (which doesn't have a
   // xul:tabbrowser), and accept HTML iframes (because tests depend on it),
@@ -594,9 +596,7 @@ GeckoDriver.prototype.registerBrowser = function(id, be) {
     this.curBrowser.register(id, be);
   }
 
-  const context = BrowsingContext.get(id);
-  this.wins.set(id, context.currentWindowGlobal);
-
+  this.wins.set(id, listenerWindow);
   return id;
 };
 
@@ -605,8 +605,8 @@ GeckoDriver.prototype.registerPromise = function() {
 
   return new Promise(resolve => {
     let cb = ({ json, target }) => {
-      let { frameId } = json;
-      this.registerBrowser(frameId, target);
+      let { outerWindowID } = json;
+      this.registerBrowser(outerWindowID, target);
 
       if (this.curBrowser.frameRegsPending > 0) {
         this.curBrowser.frameRegsPending--;
@@ -617,7 +617,7 @@ GeckoDriver.prototype.registerPromise = function() {
         resolve();
       }
 
-      return { frameId };
+      return { outerWindowID };
     };
     this.mm.addMessageListener(li, cb);
   });
@@ -628,7 +628,7 @@ GeckoDriver.prototype.listeningPromise = function() {
 
   return new Promise(resolve => {
     let cb = msg => {
-      if (msg.json.frameId === this.curBrowser.curFrameId) {
+      if (msg.json.outerWindowID === this.curBrowser.curFrameId) {
         this.mm.removeMessageListener(li, cb);
         resolve();
       }
@@ -1390,7 +1390,7 @@ GeckoDriver.prototype.getIdForBrowser = function(browser) {
     return this._browserIds.get(permKey);
   }
 
-  let winId = browser.browsingContext.id;
+  let winId = browser.outerWindowID;
   if (winId) {
     this._browserIds.set(permKey, winId);
     return winId;
@@ -1618,13 +1618,13 @@ GeckoDriver.prototype.switchToWindow = async function(cmd) {
  *     associated metadata.
  */
 GeckoDriver.prototype.findWindow = function(winIterable, filter) {
-  for (const win of winIterable) {
-    const bc = win.docShell.browsingContext;
-    const tabBrowser = browser.getTabBrowser(win);
+  for (let win of winIterable) {
+    let outerId = getOuterWindowId(win);
+    let tabBrowser = browser.getTabBrowser(win);
 
     // In case the wanted window is a chrome window, we are done.
-    if (filter(win, bc.id)) {
-      return { win, id: bc.id, hasTabBrowser: !!tabBrowser };
+    if (filter(win, outerId)) {
+      return { win, outerId, hasTabBrowser: !!tabBrowser };
 
       // Otherwise check if the chrome window has a tab browser, and that it
       // contains a tab with the wanted window handle.
@@ -1636,7 +1636,7 @@ GeckoDriver.prototype.findWindow = function(winIterable, filter) {
         if (filter(win, contentWindowId)) {
           return {
             win,
-            id: bc.id,
+            outerId,
             hasTabBrowser: true,
             tabIndex: i,
           };
@@ -1665,7 +1665,7 @@ GeckoDriver.prototype.setWindowHandle = async function(
   winProperties,
   focus = true
 ) {
-  if (!(winProperties.id in this.browsers)) {
+  if (!(winProperties.outerId in this.browsers)) {
     // Initialise Marionette if the current chrome window has not been seen
     // before. Also register the initial tab, if one exists.
     let registerBrowsers, browserListening;
@@ -1683,7 +1683,7 @@ GeckoDriver.prototype.setWindowHandle = async function(
     }
   } else {
     // Otherwise switch to the known chrome window
-    this.curBrowser = this.browsers[winProperties.id];
+    this.curBrowser = this.browsers[winProperties.outerId];
     this.mainFrame = this.curBrowser.window;
     this.curFrame = null;
 
@@ -1758,10 +1758,6 @@ GeckoDriver.prototype.switchToFrame = async function(cmd) {
 
   let { id, focus } = cmd.parameters;
 
-  if (typeof id == "number") {
-    assert.unsignedShort(id, `Expected id to be unsigned short, got ${id}`);
-  }
-
   // TODO(ato): element can be either string (deprecated) or a web
   // element JSON Object.  Can be removed with Firefox 60.
   let byFrame;
@@ -3572,12 +3568,12 @@ GeckoDriver.prototype.receiveMessage = function(message) {
       break;
 
     case "Marionette:Register":
-      let { frameId } = message.json;
-      this.registerBrowser(frameId, message.target);
-      return { frameId };
+      let { outerWindowID } = message.json;
+      this.registerBrowser(outerWindowID, message.target);
+      return { outerWindowID };
 
     case "Marionette:ListenersAttached":
-      if (message.json.frameId === this.curBrowser.curFrameId) {
+      if (message.json.outerWindowID === this.curBrowser.curFrameId) {
         this.curBrowser.flushPendingCommands();
       }
       break;
@@ -3899,8 +3895,8 @@ GeckoDriver.prototype.commands = {
   "WebDriver:TakeScreenshot": GeckoDriver.prototype.takeScreenshot,
 };
 
-function getWindowId(win) {
-  return win.docShell.browsingContext.id;
+function getOuterWindowId(win) {
+  return win.windowUtils.outerWindowID;
 }
 
 async function exitFullscreen(win) {
diff --git a/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py b/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
index 59f2af38f9e3..243c0a032328 100644
--- a/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
+++ b/testing/marionette/harness/marionette_harness/runner/mixins/window_manager.py
@@ -92,7 +92,7 @@ class WindowManagerMixin(object):
                 return self.marionette.execute_script("""
                   Components.utils.import("resource://gre/modules/Services.jsm");
 
-                  const win = BrowsingContext.get(Number(arguments[0])).window;
+                  let win = Services.wm.getOuterWindowWithId(Number(arguments[0]));
                   return win.document.readyState == "complete";
                 """, script_args=[handle])
 
@@ -174,7 +174,7 @@ class WindowManagerMixin(object):
                       await focused;
                     }
 
-                    resolve(win.docShell.browsingContext.id);
+                    resolve(win.windowUtils.outerWindowID);
                   })();
                 """, script_args=(url, focus))
 
diff --git a/testing/marionette/harness/marionette_harness/tests/unit/test_switch_frame.py b/testing/marionette/harness/marionette_harness/tests/unit/test_switch_frame.py
index 923e572bc84e..ec59b70838f0 100644
--- a/testing/marionette/harness/marionette_harness/tests/unit/test_switch_frame.py
+++ b/testing/marionette/harness/marionette_harness/tests/unit/test_switch_frame.py
@@ -6,7 +6,6 @@ from __future__ import absolute_import
 
 from marionette_driver.by import By
 from marionette_driver.errors import (
-    InvalidArgumentException,
     JavascriptException,
     NoSuchFrameException,
 )
@@ -123,13 +122,11 @@ class TestSwitchFrame(MarionetteTestCase):
     def test_switch_to_frame_with_out_of_bounds_index(self):
         self.marionette.navigate(self.marionette.absolute_url("test_iframe.html"))
         count = self.marionette.execute_script("return window.frames.length;")
-        for index in [count, 65535]:
-            self.assertRaises(NoSuchFrameException, self.marionette.switch_to_frame, index)
+        self.assertRaises(NoSuchFrameException, self.marionette.switch_to_frame, count)
 
-    def test_switch_to_frame_with_invalid_index(self):
+    def test_switch_to_frame_with_negative_index(self):
         self.marionette.navigate(self.marionette.absolute_url("test_iframe.html"))
-        for index in [-1, 65536]:
-            self.assertRaises(InvalidArgumentException, self.marionette.switch_to_frame, index)
+        self.assertRaises(NoSuchFrameException, self.marionette.switch_to_frame, -1)
 
     def test_switch_to_parent_frame(self):
         frame_html = self.marionette.absolute_url("frameset.html")
diff --git a/testing/marionette/listener.js b/testing/marionette/listener.js
index 8c04a6ba4365..18ebe963415b 100644
--- a/testing/marionette/listener.js
+++ b/testing/marionette/listener.js
@@ -58,26 +58,13 @@ const { navigate } = ChromeUtils.import(
 );
 const { proxy } = ChromeUtils.import("chrome://marionette/content/proxy.js");
 
-XPCOMUtils.defineLazyGetter(this, "logger", () => Log.getWithPrefix(contentId));
+XPCOMUtils.defineLazyGetter(this, "logger", () =>
+  Log.getWithPrefix(outerWindowID)
+);
 XPCOMUtils.defineLazyGlobalGetters(this, ["URL"]);
 
-const contentId = content.docShell.browsingContext.id;
-
-const curContainer = {
-  _frame: null,
-  shadowRoot: null,
-
-  get frame() {
-    return this._frame;
-  },
-
-  set frame(frame) {
-    this._frame = frame;
-
-    this.id = this._frame.docShell.browsingContext.id;
-    this.shadowRoot = null;
-  },
-};
+let { outerWindowID } = winUtil;
+let curContainer = { frame: content, shadowRoot: null };
 
 // Listen for click event to indicate one click has happened, so actions
 // code can send dblclick event
@@ -386,16 +373,17 @@ const loadListener = {
   },
 
   observe(subject, topic) {
-    logger.trace(`Received observer notification ${topic}`);
+    const win = curContainer.frame;
+    const winID = subject.QueryInterface(Ci.nsISupportsPRUint64).data;
+    const curWinID = win.windowUtils.outerWindowID;
 
-    const winId = subject.QueryInterface(Ci.nsISupportsPRUint64).data;
-    const bc = BrowsingContext.get(curContainer.id);
+    logger.trace(`Received observer notification ${topic}`);
 
     switch (topic) {
       // In the case when the currently selected frame is closed,
       // there will be no further load events. Stop listening immediately.
       case "outer-window-destroyed":
-        if (bc.window.windowUtils.outerWindowID == winId) {
+        if (curWinID === winID) {
           this.stop();
           sendOk(this.commandID);
         }
@@ -493,27 +481,25 @@ const loadListener = {
 function registerSelf() {
   logger.trace("Frame script loaded");
 
-  curContainer.frame = content;
-
   sandboxes.clear();
+  curContainer = {
+    frame: content,
+    shadowRoot: null,
+  };
   legacyactions.mouseEventsOnly = false;
   action.inputStateMap = new Map();
   action.inputsToCancel = [];
 
-  let reply = sendSyncMessage("Marionette:Register", {
-    frameId: contentId,
-  });
+  let reply = sendSyncMessage("Marionette:Register", { outerWindowID });
   if (reply.length == 0) {
     logger.error("No reply from Marionette:Register");
     return;
   }
 
-  if (reply[0].frameId === contentId) {
+  if (reply[0].outerWindowID === outerWindowID) {
     logger.trace("Frame script registered");
     startListeners();
-    sendAsyncMessage("Marionette:ListenersAttached", {
-      frameId: contentId,
-    });
+    sendAsyncMessage("Marionette:ListenersAttached", { outerWindowID });
   }
 }
 
@@ -680,11 +666,9 @@ function deregister() {
 
 function deleteSession() {
   seenEls.clear();
-
   // reset container frame to the top-most frame
-  curContainer.frame = content;
+  curContainer = { frame: content, shadowRoot: null };
   curContainer.frame.focus();
-
   legacyactions.touchIds = {};
   if (action.inputStateMap !== undefined) {
     action.inputStateMap.clear();
@@ -765,7 +749,8 @@ function emitTouchEvent(type, touch) {
   );
 
   const win = curContainer.frame;
-  if (win.docShell.asyncPanZoomEnabled && legacyactions.scrolling) {
+  let docShell = win.docShell;
+  if (docShell.asyncPanZoomEnabled && legacyactions.scrolling) {
     let ev = {
       index: 0,
       type,
@@ -1530,9 +1515,7 @@ function switchToParentFrame(msg) {
  */
 function switchToFrame(msg) {
   let commandID = msg.json.commandID;
-
-  let foundFrame;
-  let frameWebEl;
+  let foundFrame = null;
 
   // check if curContainer.frame reference is dead
   let frames = [];
@@ -1552,7 +1535,6 @@ function switchToFrame(msg) {
     sendSyncMessage("Marionette:switchedToFrame", { frameValue: null });
 
     curContainer.frame = content;
-
     if (msg.json.focus) {
       curContainer.frame.focus();
     }
@@ -1588,12 +1570,13 @@ function switchToFrame(msg) {
         let wrappedItem = new XPCNativeWrapper(frameEl);
         let wrappedWanted = new XPCNativeWrapper(wantedFrame);
         if (wrappedItem == wrappedWanted) {
-          foundFrame = frameEl;
+          curContainer.frame = frameEl;
+          foundFrame = i;
         }
       }
     }
 
-    if (!foundFrame) {
+    if (foundFrame === null) {
       // Either the frame has been removed or we have a OOP frame
       // so lets just get all the iframes and do a quick loop before
       // throwing in the towel
@@ -1604,47 +1587,48 @@ function switchToFrame(msg) {
         let wrappedEl = new XPCNativeWrapper(frameEl);
         let wrappedWanted = new XPCNativeWrapper(wantedFrame);
         if (wrappedEl == wrappedWanted) {
-          foundFrame = iframes[i];
+          curContainer.frame = iframes[i];
+          foundFrame = i;
         }
       }
     }
   }
 
-  if (!foundFrame) {
+  if (foundFrame === null) {
     if (typeof msg.json.id === "number") {
       try {
-        if (msg.json.id >= 0 && msg.json.id < frames.length) {
-          foundFrame = frames[msg.json.id].frameElement;
-          if (foundFrame !== null) {
-            frameWebEl = seenEls.add(foundFrame.wrappedJSObject);
-          } else {
-            // If foundFrame is null at this point then we have the top
-            // level browsing context so should treat it accordingly.
-            sendSyncMessage("Marionette:switchedToFrame", { frameValue: null });
-            curContainer.frame = content;
+        foundFrame = frames[msg.json.id].frameElement;
+        if (foundFrame !== null) {
+          curContainer.frame = foundFrame;
+          foundFrame = seenEls.add(curContainer.frame);
+        } else {
+          // If foundFrame is null at this point then we have the top
+          // level browsing context so should treat it accordingly.
+          sendSyncMessage("Marionette:switchedToFrame", { frameValue: null });
+          curContainer.frame = content;
 
-            if (msg.json.focus) {
-              curContainer.frame.focus();
-            }
-
-            sendOk(commandID);
-            return;
+          if (msg.json.focus) {
+            curContainer.frame.focus();
           }
+
+          sendOk(commandID);
+          return;
         }
       } catch (e) {
         // Since window.frames does not return OOP frames it will throw
         // and we land up here. Let's not give up and check if there are
         // iframes and switch to the indexed frame there
-        let doc = foundFrame.document;
+        let doc = curContainer.frame.document;
         let iframes = doc.getElementsByTagName("iframe");
         if (msg.json.id >= 0 && msg.json.id < iframes.length) {
-          foundFrame = iframes[msg.json.id];
+          curContainer.frame = iframes[msg.json.id];
+          foundFrame = msg.json.id;
         }
       }
     }
   }
 
-  if (!foundFrame) {
+  if (foundFrame === null) {
     let failedFrame = msg.json.id || msg.json.element;
     let err = new NoSuchFrameError(`Unable to locate frame: ${failedFrame}`);
     sendError(err, commandID);
@@ -1653,14 +1637,12 @@ function switchToFrame(msg) {
 
   // send a synchronous message to let the server update the currently active
   // frame element (for getActiveFrame)
-  if (!frameWebEl) {
-    frameWebEl = seenEls.add(foundFrame.wrappedJSObject);
-  }
+  let frameWebEl = seenEls.add(curContainer.frame.wrappedJSObject);
   sendSyncMessage("Marionette:switchedToFrame", {
     frameValue: frameWebEl.uuid,
   });
 
-  curContainer.frame = foundFrame.contentWindow;
+  curContainer.frame = curContainer.frame.contentWindow;
   if (msg.json.focus) {
     curContainer.frame.focus();
   }