--- a/browser/components/customizableui/CustomizableWidgets.jsm
+++ b/browser/components/customizableui/CustomizableWidgets.jsm
@@ -1022,20 +1022,17 @@ const CustomizableWidgets = [
         if (this.maybeDisableMenu(document))
           aNode.setAttribute("disabled", "true");
         else
           aNode.removeAttribute("disabled");
       };
 
       let getPanel = () => {
         let {PanelUI} = document.ownerGlobal;
-        if (PanelUI.overflowContents) {
-          return document.getElementById("widget-overflow");
-        }
-        return PanelUI.panel;
+        return PanelUI.overflowPanel;
       }
 
       if (CustomizableUI.getAreaType(this.currentArea) == CustomizableUI.TYPE_MENU_PANEL) {
         getPanel().addEventListener("popupshowing", updateButton);
       }
 
       let listener = {
         onWidgetAdded: (aWidgetId, aArea) => {

--- a/browser/components/customizableui/test/browser.ini
+++ b/browser/components/customizableui/test/browser.ini
@@ -79,18 +79,16 @@ skip-if = os == "linux" # Intermittent f
 skip-if = os == "linux" # Intermittent failures
 [browser_947914_button_newWindow.js]
 skip-if = os == "linux" # Intermittent failures
 [browser_947914_button_paste.js]
 subsuite = clipboard
 skip-if = os == "linux" # Intermittent failures on Linux
 [browser_947914_button_print.js]
 skip-if = os == "linux" # Intermittent failures on Linux
-[browser_947914_button_savePage.js]
-skip-if = os == "linux" # Intermittent failures
 [browser_947914_button_zoomIn.js]
 skip-if = os == "linux" # Intermittent failures
 [browser_947914_button_zoomOut.js]
 skip-if = os == "linux" # Intermittent failures
 [browser_947914_button_zoomReset.js]
 skip-if = os == "linux" # Intermittent failures
 [browser_947987_removable_default.js]
 [browser_948985_non_removable_defaultArea.js]

--- a/browser/components/customizableui/test/browser_1087303_button_fullscreen.js
+++ b/browser/components/customizableui/test/browser_1087303_button_fullscreen.js
@@ -1,19 +1,21 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check fullscreen button existence and functionality");
 
-  await PanelUI.show();
+  CustomizableUI.addWidgetToArea("fullscreen-button", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await document.getElementById("nav-bar").overflowable.show();
 
   let fullscreenButton = document.getElementById("fullscreen-button");
   ok(fullscreenButton, "Fullscreen button appears in Panel Menu");
 
   let fullscreenPromise = promiseFullscreenChange();
   fullscreenButton.click();
   await fullscreenPromise;
 

--- a/browser/components/customizableui/test/browser_1087303_button_preferences.js
+++ b/browser/components/customizableui/test/browser_1087303_button_preferences.js
@@ -2,20 +2,21 @@
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var newTab = null;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check preferences button existence and functionality");
+  CustomizableUI.addWidgetToArea("preferences-button", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
 
-  await PanelUI.show();
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let preferencesButton = document.getElementById("preferences-button");
   ok(preferencesButton, "Preferences button exists in Panel Menu");
   preferencesButton.click();
 
   newTab = gBrowser.selectedTab;
   await waitForPageLoad(newTab);

--- a/browser/components/customizableui/test/browser_947914_button_addons.js
+++ b/browser/components/customizableui/test/browser_947914_button_addons.js
@@ -3,32 +3,33 @@
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var initialLocation = gBrowser.currentURI.spec;
 var newTab = null;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
+  CustomizableUI.addWidgetToArea("add-ons-button", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
   info("Check addons button existence and functionality");
 
-  await PanelUI.show();
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let addonsButton = document.getElementById("add-ons-button");
   ok(addonsButton, "Add-ons button exists in Panel Menu");
   addonsButton.click();
 
   newTab = gBrowser.selectedTab;
   await waitForCondition(() => gBrowser.currentURI &&
                                gBrowser.currentURI.spec == "about:addons");
 
   let addonsPage = gBrowser.selectedBrowser.contentWindow.document.
                             getElementById("addons-page");
   ok(addonsPage, "Add-ons page was opened");
 });
 
 add_task(async function asyncCleanup() {
+  CustomizableUI.reset();
   BrowserTestUtils.addTab(gBrowser, initialLocation);
   gBrowser.removeTab(gBrowser.selectedTab);
   info("Tabs were restored");
 });

--- a/browser/components/customizableui/test/browser_947914_button_copy.js
+++ b/browser/components/customizableui/test/browser_947914_button_copy.js
@@ -3,36 +3,36 @@
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var initialLocation = gBrowser.currentURI.spec;
 var globalClipboard;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   await BrowserTestUtils.withNewTab({gBrowser, url: "about:blank"}, async function() {
     info("Check copy button existence and functionality");
+    CustomizableUI.addWidgetToArea("edit-controls", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
 
     let testText = "copy text test";
 
     gURLBar.focus();
     info("The URL bar was focused");
-    await PanelUI.show();
+    await document.getElementById("nav-bar").overflowable.show();
     info("Menu panel was opened");
 
     let copyButton = document.getElementById("copy-button");
     ok(copyButton, "Copy button exists in Panel Menu");
     ok(copyButton.getAttribute("disabled"), "Copy button is initially disabled");
 
     // copy text from URL bar
     gURLBar.value = testText;
     gURLBar.focus();
     gURLBar.select();
-    await PanelUI.show();
+    await document.getElementById("nav-bar").overflowable.show();
     info("Menu panel was opened");
 
     ok(!copyButton.hasAttribute("disabled"), "Copy button is enabled when selecting");
 
     copyButton.click();
     is(gURLBar.value, testText, "Selected text is unaltered when clicking copy");
 
     // check that the text was added to the clipboard
@@ -51,10 +51,11 @@ add_task(async function() {
       str.value.QueryInterface(Ci.nsISupportsString);
       clipboardValue = str.value.data;
     }
     is(clipboardValue, testText, "Data was copied to the clipboard.");
   });
 });
 
 registerCleanupFunction(function cleanup() {
+  CustomizableUI.reset();
   Services.clipboard.emptyClipboard(globalClipboard);
 });

--- a/browser/components/customizableui/test/browser_947914_button_cut.js
+++ b/browser/components/customizableui/test/browser_947914_button_cut.js
@@ -3,35 +3,35 @@
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var initialLocation = gBrowser.currentURI.spec;
 var globalClipboard;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   await BrowserTestUtils.withNewTab({gBrowser, url: "about:blank"}, async function() {
     info("Check cut button existence and functionality");
+    CustomizableUI.addWidgetToArea("edit-controls", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
 
     let testText = "cut text test";
 
     gURLBar.focus();
-    await PanelUI.show();
+    await document.getElementById("nav-bar").overflowable.show();
     info("Menu panel was opened");
 
     let cutButton = document.getElementById("cut-button");
     ok(cutButton, "Cut button exists in Panel Menu");
     ok(cutButton.hasAttribute("disabled"), "Cut button is disabled");
 
     // cut text from URL bar
     gURLBar.value = testText;
     gURLBar.focus();
     gURLBar.select();
-    await PanelUI.show();
+    await document.getElementById("nav-bar").overflowable.show();
     info("Menu panel was opened");
 
     ok(!cutButton.hasAttribute("disabled"), "Cut button is enabled when selecting");
     cutButton.click();
     is(gURLBar.value, "", "Selected text is removed from source when clicking on cut");
 
     // check that the text was added to the clipboard
     let clipboard = Services.clipboard;
@@ -49,10 +49,11 @@ add_task(async function() {
       str.value.QueryInterface(Ci.nsISupportsString);
       clipboardValue = str.value.data;
     }
     is(clipboardValue, testText, "Data was copied to the clipboard.");
   });
 });
 
 registerCleanupFunction(function cleanup() {
+  CustomizableUI.reset();
   Services.clipboard.emptyClipboard(globalClipboard);
 });

--- a/browser/components/customizableui/test/browser_947914_button_find.js
+++ b/browser/components/customizableui/test/browser_947914_button_find.js
@@ -1,19 +1,20 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check find button existence and functionality");
+  CustomizableUI.addWidgetToArea("find-button", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
 
-  await PanelUI.show();
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let findButton = document.getElementById("find-button");
   ok(findButton, "Find button exists in Panel Menu");
 
   findButton.click();
   ok(!gFindBar.hasAttribute("hidden"), "Findbar opened successfully");
 

--- a/browser/components/customizableui/test/browser_947914_button_history.js
+++ b/browser/components/customizableui/test/browser_947914_button_history.js
@@ -1,27 +1,28 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check history button existence and functionality");
+  CustomizableUI.addWidgetToArea("history-panelmenu", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
 
-  await PanelUI.show();
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let historyButton = document.getElementById("history-panelmenu");
   ok(historyButton, "History button appears in Panel Menu");
 
   let historyPanel = document.getElementById("PanelUI-history");
   let promise = BrowserTestUtils.waitForEvent(historyPanel, "ViewShown");
   historyButton.click();
   await promise;
   ok(historyPanel.getAttribute("current"), "History Panel is in view");
 
-  let panelHiddenPromise = promisePanelHidden(window);
-  PanelUI.hide();
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  document.getElementById("widget-overflow").hidePopup();
   await panelHiddenPromise
   info("Menu panel was closed");
 });

--- a/browser/components/customizableui/test/browser_947914_button_newPrivateWindow.js
+++ b/browser/components/customizableui/test/browser_947914_button_newPrivateWindow.js
@@ -1,19 +1,20 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check private browsing button existence and functionality");
+  CustomizableUI.addWidgetToArea("privatebrowsing-button", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
 
-  await PanelUI.show();
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let windowWasHandled = false;
   let privateWindow = null;
 
   let observerWindowOpened = {
     observe(aSubject, aTopic, aData) {
       if (aTopic == "domwindowopened") {

--- a/browser/components/customizableui/test/browser_947914_button_newWindow.js
+++ b/browser/components/customizableui/test/browser_947914_button_newWindow.js
@@ -1,18 +1,20 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check new window button existence and functionality");
-  await PanelUI.show();
+  CustomizableUI.addWidgetToArea("new-window-button", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let windowWasHandled = false;
   let newWindow = null;
 
   let observerWindowOpened = {
     observe(aSubject, aTopic, aData) {
       if (aTopic == "domwindowopened") {

--- a/browser/components/customizableui/test/browser_947914_button_paste.js
+++ b/browser/components/customizableui/test/browser_947914_button_paste.js
@@ -3,24 +3,24 @@
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var initialLocation = gBrowser.currentURI.spec;
 var globalClipboard;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   await BrowserTestUtils.withNewTab({gBrowser, url: "about:blank"}, async function() {
+    CustomizableUI.addWidgetToArea("edit-controls", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
     info("Check paste button existence and functionality");
 
     let clipboard = Cc["@mozilla.org/widget/clipboardhelper;1"].getService(Ci.nsIClipboardHelper);
     globalClipboard = Services.clipboard.kGlobalClipboard;
 
-    await PanelUI.show();
+    await document.getElementById("nav-bar").overflowable.show();
     info("Menu panel was opened");
 
     let pasteButton = document.getElementById("paste-button");
     ok(pasteButton, "Paste button exists in Panel Menu");
 
     // add text to clipboard
     let text = "Sample text for testing";
     clipboard.copyString(text);
@@ -33,10 +33,11 @@ add_task(async function() {
     ok(!pasteButton.hasAttribute("disabled"), "Paste button is enabled");
     pasteButton.click();
 
     is(gURLBar.value, text, "Text pasted successfully");
   });
 });
 
 registerCleanupFunction(function cleanup() {
+  CustomizableUI.reset();
   Services.clipboard.emptyClipboard(globalClipboard);
 });

--- a/browser/components/customizableui/test/browser_947914_button_print.js
+++ b/browser/components/customizableui/test/browser_947914_button_print.js
@@ -2,34 +2,35 @@
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 const isOSX = (Services.appinfo.OS === "Darwin");
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
+  CustomizableUI.addWidgetToArea("print-button", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
   await BrowserTestUtils.withNewTab({
     gBrowser,
     url: "http://example.com/",
   }, async function() {
     info("Check print button existence and functionality");
 
-    await PanelUI.show();
+    await document.getElementById("nav-bar").overflowable.show();
     info("Menu panel was opened");
 
     await waitForCondition(() => document.getElementById("print-button") != null);
 
     let printButton = document.getElementById("print-button");
     ok(printButton, "Print button exists in Panel Menu");
 
     if (isOSX) {
-      let panelHiddenPromise = promisePanelHidden(window);
-      PanelUI.hide();
+      let panelHiddenPromise = promiseOverflowHidden(window);
+      document.getElementById("widget-overflow").hidePopup();
       await panelHiddenPromise;
       info("Menu panel was closed");
     } else {
       printButton.click();
       await waitForCondition(() => gInPrintPreviewMode);
 
       ok(gInPrintPreviewMode, "Entered print preview mode");
 

deleted file mode 100644
--- a/browser/components/customizableui/test/browser_947914_button_savePage.js
+++ /dev/null
@@ -1,21 +0,0 @@
-/* This Source Code Form is subject to the terms of the Mozilla Public
-  * License, v. 2.0. If a copy of the MPL was not distributed with this
-  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
-
-"use strict";
-
-add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
-  info("Check save page button existence");
-
-  await PanelUI.show();
-  info("Menu panel was opened");
-
-  let savePageButton = document.getElementById("save-page-button");
-  ok(savePageButton, "Save Page button exists in Panel Menu");
-
-  let panelHiddenPromise = promisePanelHidden(window);
-  PanelUI.hide();
-  await panelHiddenPromise;
-  info("Menu panel was closed");
-});

--- a/browser/components/customizableui/test/browser_947914_button_zoomIn.js
+++ b/browser/components/customizableui/test/browser_947914_button_zoomIn.js
@@ -2,36 +2,38 @@
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var initialPageZoom = ZoomManager.zoom;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check zoom in button existence and functionality");
 
   is(initialPageZoom, 1, "Initial zoom factor should be 1");
 
-  await PanelUI.show();
+  CustomizableUI.addWidgetToArea("zoom-controls", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let zoomInButton = document.getElementById("zoom-in-button");
   ok(zoomInButton, "Zoom in button exists in Panel Menu");
 
   zoomInButton.click();
   let pageZoomLevel = parseInt(ZoomManager.zoom * 100);
   let zoomResetButton = document.getElementById("zoom-reset-button");
   let expectedZoomLevel = parseInt(zoomResetButton.getAttribute("label"), 10);
   ok(pageZoomLevel > 100 && pageZoomLevel == expectedZoomLevel, "Page zoomed in correctly");
 
   // close the Panel
-  let panelHiddenPromise = promisePanelHidden(window);
-  PanelUI.hide();
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  document.getElementById("widget-overflow").hidePopup();
   await panelHiddenPromise;
   info("Menu panel was closed");
 });
 
 add_task(async function asyncCleanup() {
   // reset zoom level
   ZoomManager.zoom = initialPageZoom;
   info("Zoom level was restored");

--- a/browser/components/customizableui/test/browser_947914_button_zoomOut.js
+++ b/browser/components/customizableui/test/browser_947914_button_zoomOut.js
@@ -2,37 +2,39 @@
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var initialPageZoom = ZoomManager.zoom;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check zoom out button existence and functionality");
 
   is(initialPageZoom, 1, "Initial zoom factor should be 1");
 
-  await PanelUI.show();
+  CustomizableUI.addWidgetToArea("zoom-controls", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let zoomOutButton = document.getElementById("zoom-out-button");
   ok(zoomOutButton, "Zoom out button exists in Panel Menu");
 
   zoomOutButton.click();
   let pageZoomLevel = Math.round(ZoomManager.zoom * 100);
 
   let zoomResetButton = document.getElementById("zoom-reset-button");
   let expectedZoomLevel = parseInt(zoomResetButton.getAttribute("label"), 10);
   ok(pageZoomLevel < 100 && pageZoomLevel == expectedZoomLevel, "Page zoomed out correctly");
 
   // close the panel
-  let panelHiddenPromise = promisePanelHidden(window);
-  PanelUI.hide();
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  document.getElementById("widget-overflow").hidePopup();
   await panelHiddenPromise;
   info("Menu panel was closed");
 });
 
 add_task(async function asyncCleanup() {
   // reset zoom level
   ZoomManager.zoom = initialPageZoom;
   info("Zoom level was restored");

--- a/browser/components/customizableui/test/browser_947914_button_zoomReset.js
+++ b/browser/components/customizableui/test/browser_947914_button_zoomReset.js
@@ -2,39 +2,41 @@
   * License, v. 2.0. If a copy of the MPL was not distributed with this
   * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 var initialPageZoom = ZoomManager.zoom;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check zoom reset button existence and functionality");
 
   is(initialPageZoom, 1, "Page zoom reset correctly");
   ZoomManager.zoom = 0.5;
-  await PanelUI.show();
+  CustomizableUI.addWidgetToArea("zoom-controls", CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await document.getElementById("nav-bar").overflowable.show();
   info("Menu panel was opened");
 
   let zoomResetButton = document.getElementById("zoom-reset-button");
   ok(zoomResetButton, "Zoom reset button exists in Panel Menu");
 
   zoomResetButton.click();
   await new Promise(SimpleTest.executeSoon);
 
   let pageZoomLevel = Math.floor(ZoomManager.zoom * 100);
   let expectedZoomLevel = 100;
   let buttonZoomLevel = parseInt(zoomResetButton.getAttribute("label"), 10);
   is(pageZoomLevel, expectedZoomLevel, "Page zoom reset correctly");
   is(pageZoomLevel, buttonZoomLevel, "Button displays the correct zoom level");
 
   // close the panel
-  let panelHiddenPromise = promisePanelHidden(window);
-  PanelUI.hide();
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  document.getElementById("widget-overflow").hidePopup();
   await panelHiddenPromise;
   info("Menu panel was closed");
 });
 
 add_task(async function asyncCleanup() {
   // reset zoom level
   ZoomManager.zoom = initialPageZoom;
   info("Zoom level was restored");

--- a/browser/components/customizableui/test/browser_967000_button_charEncoding.js
+++ b/browser/components/customizableui/test/browser_967000_button_charEncoding.js
@@ -2,38 +2,38 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 const TEST_PAGE = "http://mochi.test:8888/browser/browser/components/customizableui/test/support/test_967000_charEncoding_page.html";
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check Character Encoding button functionality");
 
   // add the Character Encoding button to the panel
   CustomizableUI.addWidgetToArea("characterencoding-button",
-                                  CustomizableUI.AREA_PANEL);
+                                  CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
 
-  // check the button's functionality
-  await PanelUI.show();
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await document.getElementById("nav-bar").overflowable.show();
 
   let charEncodingButton = document.getElementById("characterencoding-button");
   ok(charEncodingButton, "The Character Encoding button was added to the Panel Menu");
   is(charEncodingButton.getAttribute("disabled"), "true",
      "The Character encoding button is initially disabled");
 
-  let panelHidePromise = promisePanelHidden(window);
-  PanelUI.hide();
+  let panelHidePromise = promiseOverflowHidden(window);
+  await document.getElementById("nav-bar").overflowable._panel.hidePopup();
   await panelHidePromise;
 
   let newTab = await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PAGE, true, true);
 
-  await PanelUI.show();
+  await document.getElementById("nav-bar").overflowable.show();
   ok(!charEncodingButton.hasAttribute("disabled"), "The Character encoding button gets enabled");
   let characterEncodingView = document.getElementById("PanelUI-characterEncodingView");
   let subviewShownPromise = subviewShown(characterEncodingView);
   charEncodingButton.click();
   await subviewShownPromise;
 
   ok(characterEncodingView.hasAttribute("current"), "The Character encoding panel is displayed");
 
@@ -44,18 +44,18 @@ add_task(async function() {
 
   let checkedButtons = characterEncodingView.querySelectorAll("toolbarbutton[checked='true']");
   is(checkedButtons.length, 2, "There should be 2 checked items (1 charset, 1 detector).");
   is(checkedButtons[0].getAttribute("label"), "Unicode", "The unicode encoding is correctly selected");
   is(characterEncodingView.querySelectorAll("#PanelUI-characterEncodingView-autodetect toolbarbutton[checked='true']").length,
      1,
      "There should be 1 checked detector.");
 
-  panelHidePromise = promisePanelHidden(window);
-  PanelUI.hide();
+  panelHidePromise = promiseOverflowHidden(window);
+  await document.getElementById("nav-bar").overflowable._panel.hidePopup();
   await panelHidePromise;
 
   await BrowserTestUtils.removeTab(newTab);
 });
 
 add_task(async function asyncCleanup() {
   // reset the panel to the default state
   await resetCustomization();

--- a/browser/components/customizableui/test/browser_967000_button_feeds.js
+++ b/browser/components/customizableui/test/browser_967000_button_feeds.js
@@ -6,51 +6,50 @@
 
 const TEST_PAGE = "http://mochi.test:8888/browser/browser/components/customizableui/test/support/feeds_test_page.html";
 const TEST_FEED = "http://mochi.test:8888/browser/browser/components/customizableui/test/support/test-feed.xml"
 
 var newTab = null;
 var initialLocation = gBrowser.currentURI.spec;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
   info("Check Subscribe button functionality");
 
   // add the Subscribe button to the panel
   CustomizableUI.addWidgetToArea("feed-button",
-                                  CustomizableUI.AREA_PANEL);
+                                  CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
 
   // check the button's functionality
-  await PanelUI.show();
+  await document.getElementById("nav-bar").overflowable.show();
 
   let feedButton = document.getElementById("feed-button");
   ok(feedButton, "The Subscribe button was added to the Panel Menu");
   is(feedButton.getAttribute("disabled"), "true", "The Subscribe button is initially disabled");
 
-  let panelHidePromise = promisePanelHidden(window);
-  PanelUI.hide();
+  let panelHidePromise = promiseOverflowHidden(window);
+  await document.getElementById("nav-bar").overflowable._panel.hidePopup();
   await panelHidePromise;
 
   newTab = gBrowser.selectedTab;
   await promiseTabLoadEvent(newTab, TEST_PAGE);
 
   await PanelUI.show();
 
   await waitForCondition(() => !feedButton.hasAttribute("disabled"));
   ok(!feedButton.hasAttribute("disabled"), "The Subscribe button gets enabled");
 
   feedButton.click();
   await promiseTabLoadEvent(newTab, TEST_FEED);
 
   is(gBrowser.currentURI.spec, TEST_FEED, "Subscribe page opened");
-  ok(!isPanelUIOpen(), "Panel is closed");
+  ok(!isOverflowOpen(), "Panel is closed");
 
-  if (isPanelUIOpen()) {
-    panelHidePromise = promisePanelHidden(window);
-    PanelUI.hide();
+  if (isOverflowOpen()) {
+    panelHidePromise = promiseOverflowHidden(window);
+    await document.getElementById("nav-bar").overflowable._panel.hidePopup();
     await panelHidePromise;
   }
 });
 
 add_task(async function asyncCleanup() {
   // reset the panel UI to the default state
   await resetCustomization();
   ok(CustomizableUI.inDefaultState, "The UI is in default state again.");

--- a/browser/components/customizableui/test/browser_973641_button_addon.js
+++ b/browser/components/customizableui/test/browser_973641_button_addon.js
@@ -3,18 +3,16 @@
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 const kButton = "test_button_for_addon";
 var initialLocation = gBrowser.currentURI.spec;
 
 add_task(async function() {
-  await SpecialPowers.pushPrefEnv({set: [["browser.photon.structure.enabled", false]]});
-
   info("Check addon button functionality");
 
   // create mocked addon button on the navigation bar
   let widgetSpec = {
     id: kButton,
     type: "button",
     onClick() {
       gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, "about:addons");
@@ -28,22 +26,22 @@ add_task(async function() {
   let navBar = document.getElementById("nav-bar");
   ok(addonButton, "Addon button exists");
   ok(navBar.contains(addonButton), "Addon button is in the navbar");
   await checkButtonFunctionality(addonButton);
 
   resetTabs();
 
   // move the add-on button in the Panel Menu
-  CustomizableUI.addWidgetToArea(kButton, CustomizableUI.AREA_PANEL);
+  CustomizableUI.addWidgetToArea(kButton, CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
   ok(!navBar.contains(addonButton), "Addon button was removed from the browser bar");
 
   // check the addon button's functionality in the Panel Menu
-  await PanelUI.show();
-  var panelMenu = document.getElementById("PanelUI-mainView");
+  await document.getElementById("nav-bar").overflowable.show();
+  var panelMenu = document.getElementById("widget-overflow-mainView");
   let addonButtonInPanel = panelMenu.getElementsByAttribute("id", kButton);
   ok(panelMenu.contains(addonButton), "Addon button was added to the Panel Menu");
   await checkButtonFunctionality(addonButtonInPanel[0]);
 });
 
 add_task(async function asyncCleanup() {
   resetTabs();
 

--- a/browser/components/customizableui/test/head.js
+++ b/browser/components/customizableui/test/head.js
@@ -322,16 +322,21 @@ function promisePanelElementHidden(win, 
     aPanel.addEventListener("popuphidden", onPanelClose);
   });
 }
 
 function isPanelUIOpen() {
   return PanelUI.panel.state == "open" || PanelUI.panel.state == "showing";
 }
 
+function isOverflowOpen() {
+  let panel = document.getElementById("widget-overflow");
+  return panel.state == "open" || panel.state == "showing";
+}
+
 function subviewShown(aSubview) {
   return new Promise((resolve, reject) => {
     let win = aSubview.ownerGlobal;
     let timeoutId = win.setTimeout(() => {
       reject("Subview (" + aSubview.id + ") did not show within 20 seconds.");
     }, 20000);
     function onViewShowing(e) {
       aSubview.removeEventListener("ViewShowing", onViewShowing);