--- a/accessible/tests/mochitest/tree/test_tabbrowser.xul
+++ b/accessible/tests/mochitest/tree/test_tabbrowser.xul
@@ -85,16 +85,25 @@
             {
               // xul:toolbarbutton ("Close current tab")
               role: ROLE_PUSHBUTTON,
               children: []
             }
             );
         } else {
           SimpleTest.ok(true, "Testing Firefox tabbrowser UI.");
+          let newTabChildren = [];
+          if (SpecialPowers.getBoolPref("privacy.userContext.enabled")) {
+            newTabChildren = [
+              {
+                role: ROLE_MENUPOPUP,
+                children: []
+              }
+            ];
+          }
 
           // NB: The (3) buttons are not visible, unless manually hovered,
           //     probably due to size reduction in this test.
           tabsAccTree.children.splice(0, 0,
             {
               // xul:tab ("about:")
               role: ROLE_PAGETAB,
               children: [
@@ -114,17 +123,17 @@
                   role: ROLE_PUSHBUTTON,
                   children: []
                 }
               ]
             },
             {
               // xul:toolbarbutton ("Open a new tab")
               role: ROLE_PUSHBUTTON,
-              children: []
+              children: newTabChildren
             }
             // "List all tabs" dropdown
             // XXX: This child(?) is not present in this test.
             //      I'm not sure why (though probably expected).
             );
         }
 
         testAccessibleTree(tabBrowser().tabContainer, tabsAccTree);

--- a/browser/base/content/browser.css
+++ b/browser/base/content/browser.css
@@ -138,16 +138,26 @@ tabbrowser {
 #TabsToolbar[customizing="true"] > #tabbrowser-tabs > .tabbrowser-arrowscrollbox > .tabs-newtab-button {
   visibility: collapse;
 }
 
 #tabbrowser-tabs:not([overflow="true"])[using-closing-tabs-spacer] ~ #alltabs-button {
   visibility: hidden; /* temporary space to keep a tab's close button under the cursor */
 }
 
+.tabs-newtab-button > .toolbarbutton-menu-dropmarker,
+#new-tab-button > .toolbarbutton-menu-dropmarker {
+  display: none;
+}
+
+.tabs-newtab-button > .toolbarbutton-icon,
+#new-tab-button > .toolbarbutton-icon {
+  margin-inline-end: 0;
+}
+
 .tabbrowser-tab {
   -moz-binding: url("chrome://browser/content/tabbrowser.xml#tabbrowser-tab");
 }
 
 .tabbrowser-tab:not([pinned]) {
   -moz-box-flex: 100;
   max-width: 210px;
   min-width: 100px;
@@ -196,16 +206,17 @@ tabbrowser {
   z-index: 2;
   pointer-events: none; /* avoid blocking dragover events on scroll buttons */
 }
 
 .tabbrowser-tabs[movingtab] > .tabbrowser-tab[fadein]:not([selected]) {
   transition: transform 200ms ease-out;
 }
 
+.new-tab-popup,
 #alltabs-popup {
   -moz-binding: url("chrome://browser/content/tabbrowser.xml#tabbrowser-alltabs-popup");
 }
 
 toolbar[printpreview="true"] {
   -moz-binding: url("chrome://global/content/printPreviewBindings.xml#printpreviewtoolbar");
 }
 

--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -258,92 +258,102 @@ function UpdateBackForwardCommands(aWebN
   }
 }
 
 /**
  * Click-and-Hold implementation for the Back and Forward buttons
  * XXXmano: should this live in toolbarbutton.xml?
  */
 function SetClickAndHoldHandlers() {
-  var timer;
-
-  function openMenu(aButton) {
-    cancelHold(aButton);
-    aButton.firstChild.hidden = false;
-    aButton.open = true;
-  }
-
-  function mousedownHandler(aEvent) {
-    if (aEvent.button != 0 ||
-        aEvent.currentTarget.open ||
-        aEvent.currentTarget.disabled)
-      return;
-
-    // Prevent the menupopup from opening immediately
-    aEvent.currentTarget.firstChild.hidden = true;
-
-    aEvent.currentTarget.addEventListener("mouseout", mouseoutHandler, false);
-    aEvent.currentTarget.addEventListener("mouseup", mouseupHandler, false);
-    timer = setTimeout(openMenu, 500, aEvent.currentTarget);
-  }
-
-  function mouseoutHandler(aEvent) {
-    let buttonRect = aEvent.currentTarget.getBoundingClientRect();
-    if (aEvent.clientX >= buttonRect.left &&
-        aEvent.clientX <= buttonRect.right &&
-        aEvent.clientY >= buttonRect.bottom)
-      openMenu(aEvent.currentTarget);
-    else
-      cancelHold(aEvent.currentTarget);
-  }
-
-  function mouseupHandler(aEvent) {
-    cancelHold(aEvent.currentTarget);
-  }
-
-  function cancelHold(aButton) {
-    clearTimeout(timer);
-    aButton.removeEventListener("mouseout", mouseoutHandler, false);
-    aButton.removeEventListener("mouseup", mouseupHandler, false);
-  }
-
-  function clickHandler(aEvent) {
-    if (aEvent.button == 0 &&
-        aEvent.target == aEvent.currentTarget &&
-        !aEvent.currentTarget.open &&
-        !aEvent.currentTarget.disabled) {
-      let cmdEvent = document.createEvent("xulcommandevent");
-      cmdEvent.initCommandEvent("command", true, true, window, 0,
-                                aEvent.ctrlKey, aEvent.altKey, aEvent.shiftKey,
-                                aEvent.metaKey, null);
-      aEvent.currentTarget.dispatchEvent(cmdEvent);
-    }
-  }
-
-  function _addClickAndHoldListenersOnElement(aElm) {
-    aElm.addEventListener("mousedown", mousedownHandler, true);
-    aElm.addEventListener("click", clickHandler, true);
-  }
-
   // Bug 414797: Clone the back/forward buttons' context menu into both buttons.
   let popup = document.getElementById("backForwardMenu").cloneNode(true);
   popup.removeAttribute("id");
   // Prevent the back/forward buttons' context attributes from being inherited.
   popup.setAttribute("context", "");
 
   let backButton = document.getElementById("back-button");
   backButton.setAttribute("type", "menu");
   backButton.appendChild(popup);
-  _addClickAndHoldListenersOnElement(backButton);
+  addClickAndHoldListenersOnElement(backButton);
 
   let forwardButton = document.getElementById("forward-button");
   popup = popup.cloneNode(true);
   forwardButton.setAttribute("type", "menu");
   forwardButton.appendChild(popup);
-  _addClickAndHoldListenersOnElement(forwardButton);
+  addClickAndHoldListenersOnElement(forwardButton);
+}
+
+let holdTimersMap = new Map();
+function holdMousedownHandler(aEvent) {
+  if (aEvent.button != 0 ||
+      aEvent.currentTarget.open ||
+      aEvent.currentTarget.disabled)
+    return;
+
+  // Prevent the menupopup from opening immediately
+  aEvent.currentTarget.firstChild.hidden = true;
+
+  aEvent.currentTarget.addEventListener("mouseout", holdMouseoutHandler, false);
+  aEvent.currentTarget.addEventListener("mouseup", holdMouseupHandler, false);
+  holdTimersMap.set(aEvent.currentTarget, setTimeout(holdOpenMenu, 500, aEvent.currentTarget));
+}
+
+function holdClickHandler(aEvent) {
+  if (aEvent.button == 0 &&
+      aEvent.target == aEvent.currentTarget &&
+      !aEvent.currentTarget.open &&
+      !aEvent.currentTarget.disabled) {
+    let cmdEvent = document.createEvent("xulcommandevent");
+    cmdEvent.initCommandEvent("command", true, true, window, 0,
+                              aEvent.ctrlKey, aEvent.altKey, aEvent.shiftKey,
+                              aEvent.metaKey, null);
+    aEvent.currentTarget.dispatchEvent(cmdEvent);
+  }
+  // This is here to cancel the XUL default event
+  // dom.click() triggers a command even if there is a click handler
+  // however this can now be prevented with preventDefault().
+  aEvent.preventDefault();
+}
+
+function holdOpenMenu(aButton) {
+  cancelHold(aButton);
+  aButton.firstChild.hidden = false;
+  aButton.open = true;
+}
+
+function holdMouseoutHandler(aEvent) {
+  let buttonRect = aEvent.currentTarget.getBoundingClientRect();
+  if (aEvent.clientX >= buttonRect.left &&
+      aEvent.clientX <= buttonRect.right &&
+      aEvent.clientY >= buttonRect.bottom)
+    holdOpenMenu(aEvent.currentTarget);
+  else
+    cancelHold(aEvent.currentTarget);
+}
+
+function holdMouseupHandler(aEvent) {
+  cancelHold(aEvent.currentTarget);
+}
+
+function cancelHold(aButton) {
+  clearTimeout(holdTimersMap.get(aButton));
+  aButton.removeEventListener("mouseout", holdMouseoutHandler, false);
+  aButton.removeEventListener("mouseup", holdMouseupHandler, false);
+}
+
+function removeClickAndHoldListenersOnElement(aElm) {
+  aElm.removeEventListener("mousedown", holdMousedownHandler, true);
+  aElm.removeEventListener("click", holdClickHandler, true);
+}
+
+function addClickAndHoldListenersOnElement(aElm) {
+  holdTimersMap.delete(aElm);
+
+  aElm.addEventListener("mousedown", holdMousedownHandler, true);
+  aElm.addEventListener("click", holdClickHandler, true);
 }
 
 const gSessionHistoryObserver = {
   observe: function(subject, topic, data)
   {
     if (topic != "browser:purge-session-history")
       return;
 

--- a/browser/base/content/tabbrowser.xml
+++ b/browser/base/content/tabbrowser.xml
@@ -4934,17 +4934,17 @@
                            onmouseover="document.getBindingParent(this)._enterNewTab();"
                            onmouseout="document.getBindingParent(this)._leaveNewTab();"
                            tooltip="dynamic-shortcut-tooltip"/>
         <xul:spacer class="closing-tabs-spacer" anonid="closing-tabs-spacer"
                     style="width: 0;"/>
       </xul:arrowscrollbox>
     </content>
 
-    <implementation implements="nsIDOMEventListener">
+    <implementation implements="nsIDOMEventListener, nsIObserver">
       <constructor>
         <![CDATA[
           this.mTabClipWidth = Services.prefs.getIntPref("browser.tabs.tabClipWidth");
 
           var tab = this.firstChild;
           tab.label = this.tabbrowser.mStringBundle.getString("tabs.emptyTabTitle");
           tab.setAttribute("crop", "end");
           tab.setAttribute("onerror", "this.removeAttribute('image');");
@@ -4953,19 +4953,31 @@
           window.addEventListener("load", this, false);
 
           try {
             this._tabAnimationLoggingEnabled = Services.prefs.getBoolPref("browser.tabs.animationLogging.enabled");
           } catch (ex) {
             this._tabAnimationLoggingEnabled = false;
           }
           this._browserNewtabpageEnabled = Services.prefs.getBoolPref("browser.newtabpage.enabled");
+          this.observe(null, "nsPref:changed", "privacy.userContext.enabled");
+          Services.prefs.addObserver("privacy.userContext.enabled", this, false);
         ]]>
       </constructor>
 
+      <destructor>
+        <![CDATA[
+          Services.prefs.removeObserver("privacy.userContext.enabled", this);
+        ]]>
+      </destructor>
+
+      <field name="newtabUndoCloseTab" readonly="true">
+        document.getAnonymousElementByAttribute(this, "anonid", "newtab_undoCloseTab");
+      </field>
+
       <field name="tabbrowser" readonly="true">
         document.getElementById(this.getAttribute("tabbrowser"));
       </field>
 
       <field name="tabbox" readonly="true">
         this.tabbrowser.mTabBox;
       </field>
 
@@ -4981,16 +4993,64 @@
 
       <field name="_firstTab">null</field>
       <field name="_lastTab">null</field>
       <field name="_afterSelectedTab">null</field>
       <field name="_beforeHoveredTab">null</field>
       <field name="_afterHoveredTab">null</field>
       <field name="_hoveredTab">null</field>
 
+      <method name="observe">
+        <parameter name="aSubject"/>
+        <parameter name="aTopic"/>
+        <parameter name="aData"/>
+        <body><![CDATA[
+          switch (aTopic) {
+            case "nsPref:changed":
+              // This is the only pref observed.
+              let containersEnabled = Services.prefs.getBoolPref("privacy.userContext.enabled");
+
+              const newTab = document.getElementById("new-tab-button");
+              const newTab2 = document.getAnonymousElementByAttribute(this, "anonid", "tabs-newtab-button")
+
+              if (containersEnabled) {
+                for (let parent of [newTab, newTab2]) {
+                  if (!parent)
+                    continue;
+                  let popup = document.createElementNS(
+                                "http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul",
+                                "menupopup");
+                  if (parent.id) {
+                    popup.id = "newtab-popup";
+                  } else {
+                    popup.setAttribute("anonid", "newtab-popup");
+                  }
+                  popup.oncommand="event.stopPropagation();";
+                  popup.className = "new-tab-popup";
+                  popup.setAttribute("position", "after_end");
+                  parent.appendChild(popup);
+
+                  addClickAndHoldListenersOnElement(parent);
+                  parent.setAttribute("type", "menu");
+                }
+              } else {
+                for (let parent of [newTab, newTab2]) {
+                  if (!parent)
+                    continue;
+                  removeClickAndHoldListenersOnElement(parent);
+                  parent.removeAttribute("type");
+                  parent.firstChild.remove();
+                }
+              }
+
+              break;
+          }
+        ]]></body>
+      </method>
+
       <property name="_isCustomizing" readonly="true">
         <getter>
           let root = document.documentElement;
           return root.getAttribute("customizing") == "true" ||
                  root.getAttribute("customize-exiting") == "true";
         </getter>
       </property>
 
@@ -6668,55 +6728,64 @@
       <handler event="popupshowing">
       <![CDATA[
         if (event.target.getAttribute('id') == "alltabs_containersMenuTab") {
           createUserContextMenu(event);
           return;
         }
 
         let containersEnabled = Services.prefs.getBoolPref("privacy.userContext.enabled");
-        document.getElementById("alltabs-popup-separator-1").hidden = !containersEnabled;
-        let containersTab = document.getElementById("alltabs_containersTab");
-
-        containersTab.hidden = !containersEnabled;
-        if (PrivateBrowsingUtils.isWindowPrivate(window)) {
-          containersTab.setAttribute("disabled", "true");
+
+        if (event.target.getAttribute('anonid') == "newtab-popup" ||
+            event.target.id == "newtab-popup") {
+          createUserContextMenu(event);
+        } else {
+          document.getElementById("alltabs-popup-separator-1").hidden = !containersEnabled;
+          let containersTab = document.getElementById("alltabs_containersTab");
+
+          containersTab.hidden = !containersEnabled;
+          if (PrivateBrowsingUtils.isWindowPrivate(window)) {
+            containersTab.setAttribute("disabled", "true");
+          }
+
+          document.getElementById("alltabs_undoCloseTab").disabled =
+            SessionStore.getClosedTabCount(window) == 0;
+
+          var tabcontainer = gBrowser.tabContainer;
+
+          // Listen for changes in the tab bar.
+          tabcontainer.addEventListener("TabAttrModified", this, false);
+          tabcontainer.addEventListener("TabClose", this, false);
+          tabcontainer.mTabstrip.addEventListener("scroll", this, false);
+
+          let tabs = gBrowser.visibleTabs;
+          for (var i = 0; i < tabs.length; i++) {
+            if (!tabs[i].pinned)
+              this._createTabMenuItem(tabs[i]);
+          }
+          this._updateTabsVisibilityStatus();
         }
-
-        document.getElementById("alltabs_undoCloseTab").disabled =
-          SessionStore.getClosedTabCount(window) == 0;
-
-        var tabcontainer = gBrowser.tabContainer;
-
-        // Listen for changes in the tab bar.
-        tabcontainer.addEventListener("TabAttrModified", this, false);
-        tabcontainer.addEventListener("TabClose", this, false);
-        tabcontainer.mTabstrip.addEventListener("scroll", this, false);
-
-        let tabs = gBrowser.visibleTabs;
-        for (var i = 0; i < tabs.length; i++) {
-          if (!tabs[i].pinned)
-            this._createTabMenuItem(tabs[i]);
-        }
-        this._updateTabsVisibilityStatus();
       ]]></handler>
 
       <handler event="popuphidden">
       <![CDATA[
         if (event.target.getAttribute('id') == "alltabs_containersMenuTab") {
           return;
         }
 
         // clear out the menu popup and remove the listeners
         for (let i = this.childNodes.length - 1; i > 0; i--) {
           let menuItem = this.childNodes[i];
           if (menuItem.tab) {
             menuItem.tab.mCorrespondingMenuitem = null;
             this.removeChild(menuItem);
           }
+          if (menuItem.hasAttribute("usercontextid")) {
+            this.removeChild(menuItem);
+          }
         }
         var tabcontainer = gBrowser.tabContainer;
         tabcontainer.mTabstrip.removeEventListener("scroll", this, false);
         tabcontainer.removeEventListener("TabAttrModified", this, false);
         tabcontainer.removeEventListener("TabClose", this, false);
       ]]></handler>
 
       <handler event="DOMMenuItemActive">

--- a/browser/components/contextualidentity/test/browser/browser.ini
+++ b/browser/components/contextualidentity/test/browser/browser.ini
@@ -8,16 +8,17 @@ support-files =
   serviceworker.html
   worker.js
 
 [browser_aboutURLs.js]
 skip-if = (debug && (os == "win" || os == "linux")) # intermittent negative leak bug 1271182
 [browser_eme.js]
 [browser_favicon.js]
 [browser_forgetaboutsite.js]
+[browser_newtabButton.js]
 [browser_usercontext.js]
 [browser_usercontextid_tabdrop.js]
 skip-if = os == "mac" || os == "win" # Intermittent failure - bug 1268276
 [browser_windowName.js]
 tags = openwindow
 [browser_windowOpen.js]
 tags = openwindow
 [browser_serviceworkers.js]

new file mode 100644
--- /dev/null
+++ b/browser/components/contextualidentity/test/browser/browser_newtabButton.js
@@ -0,0 +1,34 @@
+"use strict";
+
+// Testing that when the user opens the add tab menu and clicks menu items
+// the correct context id is opened
+
+add_task(function* test() {
+  yield SpecialPowers.pushPrefEnv({"set": [
+      ["privacy.userContext.enabled", true]
+  ]});
+
+  let newTab = document.getElementById('tabbrowser-tabs');
+  let newTabButton = document.getAnonymousElementByAttribute(newTab, "anonid", "tabs-newtab-button");
+  ok(newTabButton, "New tab button exists");
+  ok(!newTabButton.hidden, "New tab button is visible");
+  let popup = document.getAnonymousElementByAttribute(newTab, "anonid", "newtab-popup");
+
+  for (let i = 1; i <= 4; i++) {
+    let popupShownPromise = BrowserTestUtils.waitForEvent(popup, "popupshown");
+    EventUtils.synthesizeMouseAtCenter(newTabButton, {type: "mousedown"});
+
+    yield popupShownPromise;
+    let contextIdItem = popup.querySelector(`menuitem[usercontextid="${i}"]`);
+
+    ok(contextIdItem, `User context id ${i} exists`);
+
+    let waitForTabPromise = BrowserTestUtils.waitForNewTab(gBrowser);
+    EventUtils.synthesizeMouseAtCenter(contextIdItem, {});
+
+    let tab = yield waitForTabPromise;
+
+    is(tab.getAttribute('usercontextid'), i, `New tab has UCI equal ${i}`);
+    yield BrowserTestUtils.removeTab(tab);
+  }
+});