--- a/browser/base/content/browser.xul
+++ b/browser/base/content/browser.xul
@@ -635,17 +635,16 @@
              collapsed="true">
 
 #ifdef CAN_DRAW_IN_TITLEBAR
       <hbox class="titlebar-placeholder" type="pre-tabs"
             skipintoolbarset="true"/>
 #endif
 
       <tabs id="tabbrowser-tabs"
-            tabbrowser="content"
             flex="1"
             setfocus="false"
             tooltip="tabbrowser-tab-tooltip"
             stopwatchid="FX_TAB_CLICK_MS">
         <tab class="tabbrowser-tab" selected="true" visuallyselected="true" fadein="true"/>
       </tabs>
 
       <toolbarbutton id="new-tab-button"

--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -6449,17 +6449,17 @@
 
       <destructor>
         <![CDATA[
           Services.prefs.removeObserver("privacy.userContext", this);
         ]]>
       </destructor>
 
       <field name="tabbrowser" readonly="true">
-        document.getElementById(this.getAttribute("tabbrowser"));
+        gBrowser;
       </field>
 
       <field name="tabbox" readonly="true">
         this.tabbrowser.tabbox;
       </field>
 
       <field name="contextMenu" readonly="true">
         document.getElementById("tabContextMenu");

--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -1332,17 +1332,17 @@ BrowserGlue.prototype = {
     let allWindowsPrivate = true;
     while (browserEnum.hasMoreElements()) {
       // XXXbz should we skip closed windows here?
       windowcount++;
 
       var browser = browserEnum.getNext();
       if (!PrivateBrowsingUtils.isWindowPrivate(browser))
         allWindowsPrivate = false;
-      var tabbrowser = browser.document.getElementById("content");
+      var tabbrowser = browser.ownerGlobal.gBrowser;
       if (tabbrowser)
         pagecount += tabbrowser.browsers.length - tabbrowser._numPinnedTabs;
     }
 
     this._saveSession = false;
     if (pagecount < 2)
       return;
 

--- a/browser/modules/FormValidationHandler.jsm
+++ b/browser/modules/FormValidationHandler.jsm
@@ -73,17 +73,17 @@ var FormValidationHandler =
   },
 
   /*
    * Internal
    */
 
   _onPopupHiding(aEvent) {
     aEvent.originalTarget.removeEventListener("popuphiding", this, true);
-    let tabBrowser = aEvent.originalTarget.ownerDocument.getElementById("content");
+    let tabBrowser = aEvent.originalTarget.ownerGlobal.gBrowser;
     tabBrowser.selectedBrowser.removeEventListener("scroll", this, true);
     tabBrowser.selectedBrowser.removeEventListener("FullZoomChange", this);
     tabBrowser.selectedBrowser.removeEventListener("TextZoomChange", this);
     tabBrowser.selectedBrowser.removeEventListener("ZoomChangeUsingMouseWheel", this);
 
     this._panel.hidden = true;
     this._panel = null;
     this._anchor.hidden = true;

--- a/devtools/client/framework/devtools-browser.js
+++ b/devtools/client/framework/devtools-browser.js
@@ -149,17 +149,17 @@ var gDevToolsBrowser = exports.gDevTools
       devtoolsTheme = "light";
     }
 
     // Style gcli and the splitter between the toolbox and page content.  This used to
     // set the attribute on the browser's root node but that regressed tpaint:
     // bug 1331449.
     win.document.getElementById("browser-bottombox")
        .setAttribute("devtoolstheme", devtoolsTheme);
-    win.document.getElementById("content")
+    win.document.getElementById("appcontent")
        .setAttribute("devtoolstheme", devtoolsTheme);
   },
 
   observe(subject, topic, prefName) {
     switch (topic) {
       case "browser-delayed-startup-finished":
         this._registerBrowserWindow(subject);
         break;

--- a/devtools/client/framework/test/browser_toolbox_theme.js
+++ b/devtools/client/framework/test/browser_toolbox_theme.js
@@ -10,23 +10,23 @@ registerCleanupFunction(() => {
   Services.prefs.clearUserPref(PREF_DEVTOOLS_THEME);
 });
 
 add_task(function* testDevtoolsTheme() {
   info("Checking stylesheet and :root attributes based on devtools theme.");
   Services.prefs.setCharPref(PREF_DEVTOOLS_THEME, "light");
   is(document.getElementById("browser-bottombox").getAttribute("devtoolstheme"), "light",
     "The element has an attribute based on devtools theme.");
-  is(document.getElementById("content").getAttribute("devtoolstheme"), "light",
+  is(document.getElementById("appcontent").getAttribute("devtoolstheme"), "light",
     "The element has an attribute based on devtools theme.");
 
   Services.prefs.setCharPref(PREF_DEVTOOLS_THEME, "dark");
   is(document.getElementById("browser-bottombox").getAttribute("devtoolstheme"), "dark",
     "The element has an attribute based on devtools theme.");
-  is(document.getElementById("content").getAttribute("devtoolstheme"), "dark",
+  is(document.getElementById("appcontent").getAttribute("devtoolstheme"), "dark",
     "The element has an attribute based on devtools theme.");
 
   Services.prefs.setCharPref(PREF_DEVTOOLS_THEME, "firebug");
   is(document.getElementById("browser-bottombox").getAttribute("devtoolstheme"), "light",
     "The element has 'light' as a default for the devtoolstheme attribute.");
-  is(document.getElementById("content").getAttribute("devtoolstheme"), "light",
+  is(document.getElementById("appcontent").getAttribute("devtoolstheme"), "light",
     "The element has 'light' as a default for the devtoolstheme attribute.");
 });

--- a/devtools/client/themes/splitters.css
+++ b/devtools/client/themes/splitters.css
@@ -15,22 +15,22 @@
   --devtools-splitter-top-width: 2px;
   --devtools-splitter-bottom-width: 2px;
 
   /* Small draggable area on inline-start to avoid overlaps on scrollbars.*/
   --devtools-splitter-inline-start-width: 1px;
   --devtools-splitter-inline-end-width: 4px;
 }
 
-#content[devtoolstheme="light"] {
+#appcontent[devtoolstheme="light"] {
   /* These variables are used in browser.xul but inside the toolbox they are overridden by --theme-splitter-color */
   --devtools-splitter-color: #dde1e4;
 }
 
-#content[devtoolstheme="dark"] {
+#appcontent[devtoolstheme="dark"] {
   --devtools-splitter-color: #42484f;
 }
 
 .devtools-horizontal-splitter,
 .devtools-side-splitter {
   -moz-appearance: none;
   background-image: none;
   border: 0;

--- a/testing/firefox-ui/tests/functional/safebrowsing/test_notification.py
+++ b/testing/firefox-ui/tests/functional/safebrowsing/test_notification.py
@@ -110,46 +110,46 @@ class TestSafeBrowsingNotificationBar(Pu
         # Clean up here since the permission gets set in this function
         self.puppeteer.utils.permissions.remove('https://www.itisatrap.org', 'safe-browsing')
 
     # Check the not a forgery or attack button in the notification bar
     def check_not_badware_button(self, button_property, report_page):
         with self.marionette.using_context('chrome'):
             # TODO: update to use safe browsing notification bar class when bug 1139544 lands
             label = self.browser.localize_property(button_property)
-            button = (self.marionette.find_element(By.ID, 'content')
+            button = (self.marionette.find_element(By.TAG_NAME, 'tabbrowser')
                       .find_element('anon attribute', {'label': label}))
 
             self.browser.tabbar.open_tab(lambda _: button.click())
 
         Wait(self.marionette, timeout=self.marionette.timeout.page_load).until(
             lambda mn: report_page in mn.get_url(),
             message='The expected safe-browsing report page has not been opened',
         )
 
         with self.marionette.using_context('chrome'):
             self.browser.tabbar.close_tab()
 
     def check_get_me_out_of_here_button(self):
         with self.marionette.using_context('chrome'):
             # TODO: update to use safe browsing notification bar class when bug 1139544 lands
             label = self.browser.localize_property('safebrowsing.getMeOutOfHereButton.label')
-            button = (self.marionette.find_element(By.ID, 'content')
+            button = (self.marionette.find_element(By.TAG_NAME, 'tabbrowser')
                       .find_element('anon attribute', {'label': label}))
             button.click()
 
         Wait(self.marionette, timeout=self.marionette.timeout.page_load).until(
             lambda mn: self.browser.default_homepage in mn.get_url(),
             message='The default home page has not been loaded',
         )
 
     def check_x_button(self):
         with self.marionette.using_context('chrome'):
             # TODO: update to use safe browsing notification bar class when bug 1139544 lands
-            button = (self.marionette.find_element(By.ID, 'content')
+            button = (self.marionette.find_element(By.TAG_NAME, 'tabbrowser')
                       .find_element('anon attribute', {'value': 'blocked-badware-page'})
                       .find_element('anon attribute',
                                     {'class': 'messageCloseButton close-icon tabbable'}))
             button.click()
 
             Wait(self.marionette, timeout=self.marionette.timeout.page_load).until(
                 expected.element_stale(button),
                 message='The notification bar has not been closed',

--- a/testing/mochitest/tests/SimpleTest/SimpleTest.js
+++ b/testing/mochitest/tests/SimpleTest/SimpleTest.js
@@ -878,17 +878,17 @@ SimpleTest.waitForFocus = function (call
     }
 
     var isWrapper = Cu.isCrossProcessWrapper(targetWindow);
     if (isWrapper || (browser && browser.isRemoteBrowser)) {
         var mustFocusSubframe = false;
         if (isWrapper) {
             // Look for a tabbrowser and see if targetWindow corresponds to one
             // within that tabbrowser. If not, just return.
-            var tabBrowser = document.getElementsByTagName("tabbrowser")[0] || null;
+            var tabBrowser = window.gBrowser || null;
             browser = tabBrowser ? tabBrowser.getBrowserForContentWindow(targetWindow.top) : null;
             if (!browser) {
                 SimpleTest.info("child process window cannot be focused");
                 return;
             }
 
             mustFocusSubframe = (targetWindow != targetWindow.top);
         }

--- a/toolkit/content/widgets/browser.xml
+++ b/toolkit/content/widgets/browser.xml
@@ -405,19 +405,20 @@
           return this._webBrowserFind;
         ]]>
         </getter>
       </property>
 
       <method name="getTabBrowser">
         <body>
           <![CDATA[
-            for (let node = this.parentNode; node instanceof Element; node = node.parentNode) {
-              if (node.localName == "tabbrowser")
-                return node;
+            if (this.ownerGlobal.gBrowser &&
+                this.ownerGlobal.gBrowser.getTabForBrowser &&
+                this.ownerGlobal.gBrowser.getTabForBrowser(this)) {
+              return this.ownerGlobal.gBrowser;
             }
             return null;
           ]]>
         </body>
       </method>
 
       <field name="_finder">null</field>