Adding upstream version 1:115.7.0.upstream/1%115.7.0upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

158 files changed, 17042 insertions, 0 deletions

diff --git a/browser/components/customizableui/test/CustomizableUITestUtils.sys.mjs b/browser/components/customizableui/test/CustomizableUITestUtils.sys.mjs
new file mode 100644
index 0000000000..2cb4e13f99
--- /dev/null
+++ b/browser/components/customizableui/test/CustomizableUITestUtils.sys.mjs
@@ -0,0 +1,156 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+/**
+ * Shared functions generally available for tests involving PanelMultiView and
+ * the CustomizableUI elements in the browser window.
+ */
+
+import { Assert } from "resource://testing-common/Assert.sys.mjs";
+
+import { BrowserTestUtils } from "resource://testing-common/BrowserTestUtils.sys.mjs";
+import { TestUtils } from "resource://testing-common/TestUtils.sys.mjs";
+
+const lazy = {};
+
+ChromeUtils.defineESModuleGetters(lazy, {
+  CustomizableUI: "resource:///modules/CustomizableUI.sys.mjs",
+});
+
+export class CustomizableUITestUtils {
+  /**
+   * Constructs an instance that operates with the specified browser window.
+   */
+  constructor(window) {
+    this.window = window;
+    this.document = window.document;
+    this.PanelUI = window.PanelUI;
+  }
+
+  /**
+   * Opens a closed PanelMultiView via the specified function while waiting for
+   * the main view with the specified ID to become fully interactive.
+   */
+  async openPanelMultiView(panel, mainView, openFn) {
+    if (panel.state == "open") {
+      // Some tests may intermittently leave the panel open. We report this, but
+      // don't fail so we don't introduce new intermittent test failures.
+      Assert.ok(
+        true,
+        "A previous test left the panel open. This should be" +
+          " fixed, but we can still do a best-effort recovery and" +
+          " assume that the requested view will be made visible."
+      );
+      await openFn();
+      return;
+    }
+
+    if (panel.state == "hiding") {
+      // There may still be tests that don't wait after invoking a command that
+      // causes the main menu panel to close. Depending on timing, the panel may
+      // or may not be fully closed when the following test runs. We handle this
+      // case gracefully so we don't risk introducing new intermittent test
+      // failures that may show up at a later time.
+      Assert.ok(
+        true,
+        "A previous test requested the panel to close but" +
+          " didn't wait for the operation to complete. While" +
+          " the test should be fixed, we can still continue."
+      );
+    } else {
+      Assert.equal(panel.state, "closed", "The panel is closed to begin with.");
+    }
+
+    let promiseShown = BrowserTestUtils.waitForEvent(mainView, "ViewShown");
+    await openFn();
+    await promiseShown;
+  }
+
+  /**
+   * Closes an open PanelMultiView via the specified function while waiting for
+   * the operation to complete.
+   */
+  async hidePanelMultiView(panel, closeFn) {
+    Assert.ok(panel.state == "open", "The panel is open to begin with.");
+
+    let promiseHidden = BrowserTestUtils.waitForEvent(panel, "popuphidden");
+    await closeFn();
+    await promiseHidden;
+  }
+
+  /**
+   * Opens the main menu and waits for it to become fully interactive.
+   */
+  async openMainMenu() {
+    await this.openPanelMultiView(
+      this.PanelUI.panel,
+      this.PanelUI.mainView,
+      () => this.PanelUI.show()
+    );
+  }
+
+  /**
+   * Closes the main menu and waits for the operation to complete.
+   */
+  async hideMainMenu() {
+    await this.hidePanelMultiView(this.PanelUI.panel, () =>
+      this.PanelUI.hide()
+    );
+  }
+
+  /**
+   * Add the search bar into the nav bar and verify it does not overflow.
+   *
+   * @returns {Promise}
+   * @resolves The search bar element.
+   * @rejects If search bar is not found, or overflows.
+   */
+  async addSearchBar() {
+    lazy.CustomizableUI.addWidgetToArea(
+      "search-container",
+      lazy.CustomizableUI.AREA_NAVBAR,
+      lazy.CustomizableUI.getPlacementOfWidget("urlbar-container").position + 1
+    );
+
+    // addWidgetToArea adds the search bar into the nav bar first.  If the
+    // search bar overflows, OverflowableToolbar for the nav bar moves the
+    // search bar into the overflow panel in its overflow event handler
+    // asynchronously.
+    //
+    // We should first wait for the layout flush to make sure either the search
+    // bar fits into the nav bar, or overflow event gets dispatched and the
+    // overflow event handler is called.
+    await this.window.promiseDocumentFlushed(() => {});
+
+    // Check if the OverflowableToolbar is handling the overflow event.
+    let navbar = this.window.document.getElementById(
+      lazy.CustomizableUI.AREA_NAVBAR
+    );
+    await TestUtils.waitForCondition(() => {
+      return !navbar.overflowable.isHandlingOverflow();
+    });
+
+    let searchbar = this.window.document.getElementById("searchbar");
+    if (!searchbar) {
+      throw new Error("The search bar should exist.");
+    }
+
+    // If the search bar overflows, it's placed inside the overflow panel.
+    //
+    // We cannot use navbar's property to check if overflow happens, since it
+    // can be different widget than the search bar that overflows.
+    if (searchbar.closest("#widget-overflow")) {
+      throw new Error(
+        "The search bar should not overflow from the nav bar. " +
+          "This test fails if the screen resolution is small and " +
+          "the search bar overflows from the nav bar."
+      );
+    }
+
+    return searchbar;
+  }
+
+  removeSearchBar() {
+    lazy.CustomizableUI.removeWidgetFromArea("search-container");
+  }
+}
diff --git a/browser/components/customizableui/test/browser.ini b/browser/components/customizableui/test/browser.ini
new file mode 100644
index 0000000000..180f8ebde2
--- /dev/null
+++ b/browser/components/customizableui/test/browser.ini
@@ -0,0 +1,210 @@
+[DEFAULT]
+support-files =
+  head.js
+  support/test_967000_charEncoding_page.html
+
+[browser_1003588_no_specials_in_panel.js]
+[browser_1008559_anchor_undo_restore.js]
+[browser_1042100_default_placements_update.js]
+[browser_1058573_showToolbarsDropdown.js]
+[browser_1087303_button_fullscreen.js]
+tags = fullscreen
+skip-if = os == "mac"
+[browser_1087303_button_preferences.js]
+[browser_1089591_still_customizable_after_reset.js]
+[browser_1096763_seen_widgets_post_reset.js]
+[browser_1161838_inserted_new_default_buttons.js]
+skip-if = verify
+[browser_1484275_PanelMultiView_toggle_with_other_popup.js]
+[browser_1701883_restore_defaults_pocket_pref.js]
+[browser_1702200_PanelMultiView_header_separator.js]
+[browser_1795260_searchbar_overflow_toolbar.js]
+tags = overflowable-toolbar
+[browser_694291_searchbar_preference.js]
+[browser_873501_handle_specials.js]
+[browser_876926_customize_mode_wrapping.js]
+skip-if =
+  (os == "linux") && !debug # Bug 1682752
+[browser_876944_customize_mode_create_destroy.js]
+[browser_877006_missing_view.js]
+[browser_877178_unregisterArea.js]
+[browser_877447_skip_missing_ids.js]
+[browser_878452_drag_to_panel.js]
+[browser_884402_customize_from_overflow.js]
+tags = overflowable-toolbar
+[browser_885052_customize_mode_observers_disabed.js]
+tags = fullscreen
+
+[browser_885530_showInPrivateBrowsing.js]
+[browser_886323_buildArea_removable_nodes.js]
+[browser_890262_destroyWidget_after_add_to_panel.js]
+[browser_892955_isWidgetRemovable_for_removed_widgets.js]
+[browser_892956_destroyWidget_defaultPlacements.js]
+[browser_901207_searchbar_in_panel.js]
+[browser_909779_overflow_toolbars_new_window.js]
+tags = overflowable-toolbar
+skip-if = os == "linux"
+
+[browser_913972_currentset_overflow.js]
+tags = overflowable-toolbar
+[browser_914138_widget_API_overflowable_toolbar.js]
+tags = overflowable-toolbar
+skip-if = os == "linux"
+
+[browser_918049_skipintoolbarset_dnd.js]
+[browser_923857_customize_mode_event_wrapping_during_reset.js]
+[browser_927717_customize_drag_empty_toolbar.js]
+[browser_934113_menubar_removable.js]
+# Because this test is about the menubar, it can't be run on mac
+skip-if = os == "mac"
+
+[browser_934951_zoom_in_toolbar.js]
+skip-if =
+  os == "win" && os_version == "6.1" # Skip on Azure - frequent failure
+[browser_938980_navbar_collapsed.js]
+[browser_938995_indefaultstate_nonremovable.js]
+[browser_940013_registerToolbarNode_calls_registerArea.js]
+[browser_940307_panel_click_closure_handling.js]
+skip-if = (verify && debug && (os == 'linux'))
+[browser_940946_removable_from_navbar_customizemode.js]
+[browser_941083_invalidate_wrapper_cache_createWidget.js]
+skip-if = verify
+[browser_942581_unregisterArea_keeps_placements.js]
+[browser_944887_destroyWidget_should_destroy_in_palette.js]
+[browser_945739_showInPrivateBrowsing_customize_mode.js]
+[browser_947914_button_copy.js]
+[browser_947914_button_cut.js]
+[browser_947914_button_find.js]
+[browser_947914_button_history.js]
+https_first_disabled = true
+support-files = dummy_history_item.html
+[browser_947914_button_newPrivateWindow.js]
+[browser_947914_button_newWindow.js]
+[browser_947914_button_paste.js]
+[browser_947914_button_print.js]
+[browser_947914_button_zoomIn.js]
+[browser_947914_button_zoomOut.js]
+[browser_947914_button_zoomReset.js]
+skip-if = (os == "linux" && debug) # Intermittent failures
+[browser_947987_removable_default.js]
+[browser_948985_non_removable_defaultArea.js]
+[browser_952963_areaType_getter_no_area.js]
+skip-if = verify
+[browser_956602_remove_special_widget.js]
+[browser_962069_drag_to_overflow_chevron.js]
+tags = overflowable-toolbar
+[browser_963639_customizing_attribute_non_customizable_toolbar.js]
+[browser_968565_insert_before_hidden_items.js]
+[browser_969427_recreate_destroyed_widget_after_reset.js]
+[browser_969661_character_encoding_navbar_disabled.js]
+[browser_970511_undo_restore_default.js]
+skip-if = verify
+[browser_972267_customizationchange_events.js]
+[browser_976792_insertNodeInWindow.js]
+tags = overflowable-toolbar
+skip-if = os == "linux"
+[browser_978084_dragEnd_after_move.js]
+skip-if = verify
+[browser_980155_add_overflow_toolbar.js]
+tags = overflowable-toolbar
+skip-if = verify
+[browser_981305_separator_insertion.js]
+[browser_981418-widget-onbeforecreated-handler.js]
+skip-if = verify
+[browser_982656_restore_defaults_builtin_widgets.js]
+[browser_984455_bookmarks_items_reparenting.js]
+[browser_985815_propagate_setToolbarVisibility.js]
+[browser_987177_destroyWidget_xul.js]
+skip-if = verify
+[browser_987177_xul_wrapper_updating.js]
+[browser_987492_window_api.js]
+[browser_987640_charEncoding.js]
+[browser_989338_saved_placements_not_resaved.js]
+[browser_989751_subviewbutton_class.js]
+[browser_992747_toggle_noncustomizable_toolbar.js]
+[browser_993322_widget_notoolbar.js]
+skip-if = verify
+[browser_995164_registerArea_during_customize_mode.js]
+[browser_996364_registerArea_different_properties.js]
+[browser_996635_remove_non_widgets.js]
+
+# Unit tests for the PanelMultiView module. These are independent from
+# CustomizableUI, but are located here together with the module they're testing.
+[browser_PanelMultiView.js]
+[browser_PanelMultiView_focus.js]
+[browser_PanelMultiView_keyboard.js]
+[browser_addons_area.js]
+[browser_allow_dragging_removable_false.js]
+[browser_backfwd_enabled_post_customize.js]
+[browser_bookmarks_empty_message.js]
+[browser_bookmarks_toolbar_collapsed_restore_default.js]
+[browser_bookmarks_toolbar_shown_newtab.js]
+[browser_bootstrapped_custom_toolbar.js]
+[browser_check_tooltips_in_navbar.js]
+[browser_create_button_widget.js]
+[browser_ctrl_click_panel_opening.js]
+[browser_currentset_post_reset.js]
+[browser_customization_context_menus.js]
+[browser_customizemode_contextmenu_menubuttonstate.js]
+[browser_customizemode_lwthemes.js]
+[browser_customizemode_uidensity.js]
+[browser_disable_commands_customize.js]
+[browser_drag_outside_palette.js]
+[browser_editcontrols_update.js]
+[browser_exit_background_customize_mode.js]
+https_first_disabled = true
+[browser_flexible_space_area.js]
+[browser_help_panel_cloning.js]
+[browser_hidden_widget_overflow.js]
+[browser_history_after_appMenu.js]
+[browser_history_recently_closed.js]
+[browser_history_recently_closed_middleclick.js]
+https_first_disabled = true
+skip-if =
+  os == "win" && os_version == "6.1" # Skip on Azure - frequent failure
+[browser_history_restore_session.js]
+[browser_insert_before_moved_node.js]
+[browser_menubar_visibility.js]
+skip-if = os == "mac" # no toggle-able menubar on macOS.
+[browser_newtab_button_customizemode.js]
+[browser_open_from_popup.js]
+[browser_open_in_lazy_tab.js]
+[browser_overflow_use_subviews.js]
+tags = overflowable-toolbar
+skip-if = verify
+[browser_palette_labels.js]
+[browser_panelUINotifications.js]
+[browser_panelUINotifications_bannerVisibility.js]
+[browser_panelUINotifications_fullscreen.js]
+tags = fullscreen
+skip-if = os == "mac"
+[browser_panelUINotifications_fullscreen_noAutoHideToolbar.js]
+skip-if = (verify && (os == 'linux' || os == 'mac'))
+tags = fullscreen
+[browser_panelUINotifications_modals.js]
+[browser_panelUINotifications_multiWindow.js]
+[browser_panel_keyboard_navigation.js]
+[browser_panel_locationSpecific.js]
+[browser_panel_toggle.js]
+[browser_proton_moreTools_panel.js]
+[browser_proton_toolbar_hide_toolbarbuttons.js]
+[browser_registerArea.js]
+[browser_reload_tab.js]
+[browser_remote_attribute.js]
+[browser_remote_tabs_button.js]
+skip-if = (verify && debug && (os == 'linux' || os == 'mac'))
+[browser_remove_customized_specials.js]
+[browser_reset_builtin_widget_currentArea.js]
+[browser_reset_dom_events.js]
+[browser_screenshot_button_disabled.js]
+[browser_sidebar_toggle.js]
+skip-if = verify
+[browser_switch_to_customize_mode.js]
+[browser_synced_tabs_menu.js]
+[browser_tabbar_big_widgets.js]
+[browser_toolbar_collapsed_states.js]
+[browser_touchbar_customization.js]
+skip-if = (os == "linux" || os == "win")
+[browser_unified_extensions_reset.js]
+[browser_widget_animation.js]
+[browser_widget_recreate_events.js]
diff --git a/browser/components/customizableui/test/browser_1003588_no_specials_in_panel.js b/browser/components/customizableui/test/browser_1003588_no_specials_in_panel.js
new file mode 100644
index 0000000000..5aa2860827
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1003588_no_specials_in_panel.js
@@ -0,0 +1,133 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+function simulateItemDragAndEnd(aToDrag, aTarget) {
+  var ds = Cc["@mozilla.org/widget/dragservice;1"].getService(
+    Ci.nsIDragService
+  );
+
+  ds.startDragSessionForTests(
+    Ci.nsIDragService.DRAGDROP_ACTION_MOVE |
+      Ci.nsIDragService.DRAGDROP_ACTION_COPY |
+      Ci.nsIDragService.DRAGDROP_ACTION_LINK
+  );
+  try {
+    var [result, dataTransfer] = EventUtils.synthesizeDragOver(
+      aToDrag.parentNode,
+      aTarget
+    );
+    EventUtils.synthesizeDropAfterDragOver(result, dataTransfer, aTarget);
+    // Send dragend to move dragging item back to initial place.
+    EventUtils.sendDragEvent(
+      { type: "dragend", dataTransfer },
+      aToDrag.parentNode
+    );
+  } finally {
+    ds.endDragSession(true);
+  }
+}
+
+add_task(async function checkNoAddingToPanel() {
+  let area = CustomizableUI.AREA_FIXED_OVERFLOW_PANEL;
+  let previousPlacements = getAreaWidgetIds(area);
+  CustomizableUI.addWidgetToArea("separator", area);
+  CustomizableUI.addWidgetToArea("spring", area);
+  CustomizableUI.addWidgetToArea("spacer", area);
+  assertAreaPlacements(area, previousPlacements);
+
+  let oldNumberOfItems = previousPlacements.length;
+  if (getAreaWidgetIds(area).length != oldNumberOfItems) {
+    CustomizableUI.reset();
+  }
+});
+
+add_task(async function checkAddingToToolbar() {
+  let area = CustomizableUI.AREA_NAVBAR;
+  let previousPlacements = getAreaWidgetIds(area);
+  CustomizableUI.addWidgetToArea("separator", area);
+  CustomizableUI.addWidgetToArea("spring", area);
+  CustomizableUI.addWidgetToArea("spacer", area);
+  let expectedPlacements = [...previousPlacements].concat([
+    /separator/,
+    /spring/,
+    /spacer/,
+  ]);
+  assertAreaPlacements(area, expectedPlacements);
+
+  let newlyAddedElements = getAreaWidgetIds(area).slice(-3);
+  while (newlyAddedElements.length) {
+    CustomizableUI.removeWidgetFromArea(newlyAddedElements.shift());
+  }
+
+  assertAreaPlacements(area, previousPlacements);
+
+  let oldNumberOfItems = previousPlacements.length;
+  if (getAreaWidgetIds(area).length != oldNumberOfItems) {
+    CustomizableUI.reset();
+  }
+});
+
+add_task(async function checkDragging() {
+  let startArea = CustomizableUI.AREA_TABSTRIP;
+  let targetArea = CustomizableUI.AREA_FIXED_OVERFLOW_PANEL;
+  let startingToolbarPlacements = getAreaWidgetIds(startArea);
+  let startingTargetPlacements = getAreaWidgetIds(targetArea);
+
+  CustomizableUI.addWidgetToArea("separator", startArea);
+  CustomizableUI.addWidgetToArea("spring", startArea);
+  CustomizableUI.addWidgetToArea("spacer", startArea);
+
+  let placementsWithSpecials = getAreaWidgetIds(startArea);
+  let elementsToMove = [];
+  for (let id of placementsWithSpecials) {
+    if (CustomizableUI.isSpecialWidget(id)) {
+      elementsToMove.push(id);
+    }
+  }
+  is(elementsToMove.length, 3, "Should have 3 elements to try and drag.");
+
+  await startCustomizing();
+  let existingSpecial = null;
+  existingSpecial =
+    gCustomizeMode.visiblePalette.querySelector("toolbarspring");
+  ok(
+    existingSpecial,
+    "Should have a flexible space in the palette by default in photon"
+  );
+  for (let id of elementsToMove) {
+    simulateItemDragAndEnd(
+      document.getElementById(id),
+      document.getElementById(targetArea)
+    );
+  }
+
+  assertAreaPlacements(startArea, placementsWithSpecials);
+  assertAreaPlacements(targetArea, startingTargetPlacements);
+
+  for (let id of elementsToMove) {
+    simulateItemDrag(
+      document.getElementById(id),
+      gCustomizeMode.visiblePalette
+    );
+  }
+
+  assertAreaPlacements(startArea, startingToolbarPlacements);
+  assertAreaPlacements(targetArea, startingTargetPlacements);
+
+  let allSpecials = gCustomizeMode.visiblePalette.querySelectorAll(
+    "toolbarspring,toolbarseparator,toolbarspacer"
+  );
+  allSpecials = [...allSpecials].filter(special => special != existingSpecial);
+  ok(
+    !allSpecials.length,
+    "No (new) specials should make it to the palette alive."
+  );
+  await endCustomizing();
+});
+
+add_task(async function asyncCleanup() {
+  await endCustomizing();
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_1008559_anchor_undo_restore.js b/browser/components/customizableui/test/browser_1008559_anchor_undo_restore.js
new file mode 100644
index 0000000000..a7da97cc95
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1008559_anchor_undo_restore.js
@@ -0,0 +1,102 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const kAnchorAttribute = "cui-anchorid";
+
+/**
+ * Check that anchor gets set correctly when moving an item from the panel to the toolbar
+ * and into the palette.
+ */
+add_task(async function () {
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  await startCustomizing();
+  let button = document.getElementById("history-panelmenu");
+  is(
+    button.getAttribute(kAnchorAttribute),
+    "nav-bar-overflow-button",
+    "Button (" + button.id + ") starts out with correct anchor"
+  );
+
+  let navbar = CustomizableUI.getCustomizationTarget(
+    document.getElementById("nav-bar")
+  );
+  let onMouseUp = BrowserTestUtils.waitForEvent(navbar, "mouseup");
+  simulateItemDrag(button, navbar);
+  await onMouseUp;
+  is(
+    CustomizableUI.getPlacementOfWidget(button.id).area,
+    "nav-bar",
+    "Button (" + button.id + ") ends up in nav-bar"
+  );
+
+  ok(
+    !button.hasAttribute(kAnchorAttribute),
+    "Button (" + button.id + ") has no anchor in toolbar"
+  );
+
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  is(
+    button.getAttribute(kAnchorAttribute),
+    "nav-bar-overflow-button",
+    "Button (" + button.id + ") has anchor again"
+  );
+
+  let resetButton = document.getElementById("customization-reset-button");
+  ok(!resetButton.hasAttribute("disabled"), "Should be able to reset now.");
+  await gCustomizeMode.reset();
+
+  ok(
+    !button.hasAttribute(kAnchorAttribute),
+    "Button (" + button.id + ") once again has no anchor in customize panel"
+  );
+
+  await endCustomizing();
+});
+
+/**
+ * Check that anchor gets set correctly when moving an item from the panel to the toolbar
+ * using 'reset'
+ */
+add_task(async function () {
+  await startCustomizing();
+  let button = document.getElementById("stop-reload-button");
+  ok(
+    !button.hasAttribute(kAnchorAttribute),
+    "Button (" + button.id + ") has no anchor in toolbar"
+  );
+
+  let panel = document.getElementById(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  let onMouseUp = BrowserTestUtils.waitForEvent(panel, "mouseup");
+  simulateItemDrag(button, panel);
+  await onMouseUp;
+  is(
+    CustomizableUI.getPlacementOfWidget(button.id).area,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+    "Button (" + button.id + ") ends up in panel"
+  );
+  is(
+    button.getAttribute(kAnchorAttribute),
+    "nav-bar-overflow-button",
+    "Button (" + button.id + ") has correct anchor in the panel"
+  );
+
+  let resetButton = document.getElementById("customization-reset-button");
+  ok(!resetButton.hasAttribute("disabled"), "Should be able to reset now.");
+  await gCustomizeMode.reset();
+
+  ok(
+    !button.hasAttribute(kAnchorAttribute),
+    "Button (" + button.id + ") once again has no anchor in toolbar"
+  );
+
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_1042100_default_placements_update.js b/browser/components/customizableui/test/browser_1042100_default_placements_update.js
new file mode 100644
index 0000000000..c20546c300
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1042100_default_placements_update.js
@@ -0,0 +1,231 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+function getSavedStatePlacements(area) {
+  return CustomizableUI.getTestOnlyInternalProp("gSavedState").placements[area];
+}
+
+// NB: This uses some ugly hacks to get into the CUI module from elsewhere...
+// don't try this at home, kids.
+function test() {
+  // Customize something to make sure stuff changed:
+  CustomizableUI.addWidgetToArea(
+    "save-page-button",
+    CustomizableUI.AREA_NAVBAR
+  );
+
+  let oldState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+  registerCleanupFunction(() =>
+    CustomizableUI.setTestOnlyInternalProp("gSavedState", oldState)
+  );
+
+  let gFuturePlacements =
+    CustomizableUI.getTestOnlyInternalProp("gFuturePlacements");
+  is(
+    gFuturePlacements.size,
+    0,
+    "All future placements should be dealt with by now."
+  );
+
+  let gPalette = CustomizableUI.getTestOnlyInternalProp("gPalette");
+  let CustomizableUIInternal = CustomizableUI.getTestOnlyInternalProp(
+    "CustomizableUIInternal"
+  );
+  CustomizableUIInternal._updateForNewVersion();
+  is(gFuturePlacements.size, 0, "No change to future placements initially.");
+
+  let currentVersion = CustomizableUI.getTestOnlyInternalProp("kVersion");
+
+  // Add our widget to the defaults:
+  let testWidgetNew = {
+    id: "test-messing-with-default-placements-new",
+    label: "Test messing with default placements - should be inserted",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+    introducedInVersion: currentVersion + 1,
+  };
+
+  let normalizedWidget = CustomizableUIInternal.normalizeWidget(
+    testWidgetNew,
+    CustomizableUI.SOURCE_BUILTIN
+  );
+  ok(normalizedWidget, "Widget should be normalizable");
+  if (!normalizedWidget) {
+    return;
+  }
+  gPalette.set(testWidgetNew.id, normalizedWidget);
+
+  let testWidgetOld = {
+    id: "test-messing-with-default-placements-old",
+    label: "Test messing with default placements - should NOT be inserted",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+    introducedInVersion: currentVersion,
+  };
+
+  normalizedWidget = CustomizableUIInternal.normalizeWidget(
+    testWidgetOld,
+    CustomizableUI.SOURCE_BUILTIN
+  );
+  ok(normalizedWidget, "Widget should be normalizable");
+  if (!normalizedWidget) {
+    return;
+  }
+  gPalette.set(testWidgetOld.id, normalizedWidget);
+
+  // Now increase the version in the module:
+  CustomizableUI.setTestOnlyInternalProp(
+    "kVersion",
+    CustomizableUI.getTestOnlyInternalProp("kVersion") + 1
+  );
+
+  let hadSavedState = !!CustomizableUI.getTestOnlyInternalProp("gSavedState");
+  if (!hadSavedState) {
+    CustomizableUI.setTestOnlyInternalProp("gSavedState", {
+      currentVersion: CustomizableUI.getTestOnlyInternalProp("kVersion") - 1,
+    });
+  }
+
+  // Then call the re-init routine so we re-add the builtin widgets
+  CustomizableUIInternal._updateForNewVersion();
+  is(gFuturePlacements.size, 1, "Should have 1 more future placement");
+  let haveNavbarPlacements = gFuturePlacements.has(CustomizableUI.AREA_NAVBAR);
+  ok(haveNavbarPlacements, "Should have placements for nav-bar");
+  if (haveNavbarPlacements) {
+    let placements = [...gFuturePlacements.get(CustomizableUI.AREA_NAVBAR)];
+
+    // Ignore widgets that are placed using the pref facility and not the
+    // versioned facility.  They're independent of kVersion and the saved
+    // state's current version, so they may be present in the placements.
+    for (let i = 0; i < placements.length; ) {
+      if (placements[i] == testWidgetNew.id) {
+        i++;
+        continue;
+      }
+      let pref = "browser.toolbarbuttons.introduced." + placements[i];
+      let introduced = Services.prefs.getBoolPref(pref, false);
+      if (!introduced) {
+        i++;
+        continue;
+      }
+      placements.splice(i, 1);
+    }
+
+    is(placements.length, 1, "Should have 1 newly placed widget in nav-bar");
+    is(
+      placements[0],
+      testWidgetNew.id,
+      "Should have our test widget to be placed in nav-bar"
+    );
+  }
+
+  // Reset kVersion
+  CustomizableUI.setTestOnlyInternalProp(
+    "kVersion",
+    CustomizableUI.getTestOnlyInternalProp("kVersion") - 1
+  );
+
+  // Now test that the builtin photon migrations work:
+
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", {
+    currentVersion: 6,
+    placements: {
+      "nav-bar": ["urlbar-container", "bookmarks-menu-button"],
+      "PanelUI-contents": ["panic-button", "edit-controls"],
+    },
+  });
+  Services.prefs.setIntPref("browser.proton.toolbar.version", 0);
+  CustomizableUIInternal._updateForNewVersion();
+  CustomizableUIInternal._updateForNewProtonVersion();
+  {
+    let navbarPlacements = getSavedStatePlacements("nav-bar");
+    let springs = navbarPlacements.filter(id => id.includes("spring"));
+    is(springs.length, 2, "Should have 2 toolbarsprings in placements now");
+    navbarPlacements = navbarPlacements.filter(id => !id.includes("spring"));
+    Assert.deepEqual(
+      navbarPlacements,
+      [
+        "back-button",
+        "forward-button",
+        "stop-reload-button",
+        "urlbar-container",
+        "downloads-button",
+        "fxa-toolbar-menu-button",
+      ],
+      "Nav-bar placements should be correct."
+    );
+
+    Assert.deepEqual(getSavedStatePlacements("widget-overflow-fixed-list"), [
+      "panic-button",
+    ]);
+  }
+
+  // Finally, test the downloads and fxa avatar button migrations work.
+  let oldNavbarPlacements = [
+    "urlbar-container",
+    "customizableui-special-spring3",
+    "search-container",
+  ];
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", {
+    currentVersion: 10,
+    placements: {
+      "nav-bar": Array.from(oldNavbarPlacements),
+      "widget-overflow-fixed-list": ["downloads-button"],
+    },
+  });
+  CustomizableUIInternal._updateForNewVersion();
+  Assert.deepEqual(
+    getSavedStatePlacements("nav-bar"),
+    oldNavbarPlacements.concat(["downloads-button", "fxa-toolbar-menu-button"]),
+    "Downloads button inserted in navbar"
+  );
+  Assert.deepEqual(
+    getSavedStatePlacements("widget-overflow-fixed-list"),
+    [],
+    "Overflow panel is empty"
+  );
+
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", {
+    currentVersion: 10,
+    placements: {
+      "nav-bar": ["downloads-button"].concat(oldNavbarPlacements),
+    },
+  });
+  CustomizableUIInternal._updateForNewVersion();
+  Assert.deepEqual(
+    getSavedStatePlacements("nav-bar"),
+    oldNavbarPlacements.concat(["downloads-button", "fxa-toolbar-menu-button"]),
+    "Downloads button reinserted in navbar"
+  );
+
+  oldNavbarPlacements = [
+    "urlbar-container",
+    "customizableui-special-spring3",
+    "search-container",
+    "other-widget",
+  ];
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", {
+    currentVersion: 10,
+    placements: {
+      "nav-bar": Array.from(oldNavbarPlacements),
+    },
+  });
+  CustomizableUIInternal._updateForNewVersion();
+  let expectedNavbarPlacements = [
+    "urlbar-container",
+    "customizableui-special-spring3",
+    "search-container",
+    "downloads-button",
+    "other-widget",
+    "fxa-toolbar-menu-button",
+  ];
+  Assert.deepEqual(
+    getSavedStatePlacements("nav-bar"),
+    expectedNavbarPlacements,
+    "Downloads button inserted in navbar before other widgets"
+  );
+
+  gFuturePlacements.delete(CustomizableUI.AREA_NAVBAR);
+  gPalette.delete(testWidgetNew.id);
+  gPalette.delete(testWidgetOld.id);
+}
diff --git a/browser/components/customizableui/test/browser_1058573_showToolbarsDropdown.js b/browser/components/customizableui/test/browser_1058573_showToolbarsDropdown.js
new file mode 100644
index 0000000000..0e57ef8a28
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1058573_showToolbarsDropdown.js
@@ -0,0 +1,29 @@
+/* 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 () {
+  info("Check that toggleable toolbars dropdown in always shown");
+
+  info("Remove all possible custom toolbars");
+  await removeCustomToolbars();
+
+  info("Enter customization mode");
+  await startCustomizing();
+
+  let toolbarsToggle = document.getElementById(
+    "customization-toolbar-visibility-button"
+  );
+  ok(toolbarsToggle, "The toolbars toggle dropdown exists");
+  ok(
+    !toolbarsToggle.hasAttribute("hidden"),
+    "The toolbars toggle dropdown is displayed"
+  );
+});
+
+add_task(async function asyncCleanup() {
+  info("Exit customization mode");
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_1087303_button_fullscreen.js b/browser/components/customizableui/test/browser_1087303_button_fullscreen.js
new file mode 100644
index 0000000000..f67e81b892
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1087303_button_fullscreen.js
@@ -0,0 +1,55 @@
+/* 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/. */
+/* eslint-disable mozilla/no-arbitrary-setTimeout */
+
+"use strict";
+
+add_task(async function () {
+  info("Check fullscreen button existence and functionality");
+
+  CustomizableUI.addWidgetToArea(
+    "fullscreen-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await waitForOverflowButtonShown();
+
+  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;
+
+  ok(window.fullScreen, "Fullscreen mode was opened");
+
+  // exit full screen mode
+  fullscreenPromise = promiseFullscreenChange();
+  window.fullScreen = !window.fullScreen;
+  await fullscreenPromise;
+
+  ok(!window.fullScreen, "Successfully exited fullscreen");
+});
+
+function promiseFullscreenChange() {
+  return new Promise((resolve, reject) => {
+    info("Wait for fullscreen change");
+
+    let timeoutId = setTimeout(() => {
+      window.removeEventListener("fullscreen", onFullscreenChange, true);
+      reject("Fullscreen change did not happen within " + 20000 + "ms");
+    }, 20000);
+
+    function onFullscreenChange(event) {
+      clearTimeout(timeoutId);
+      window.removeEventListener("fullscreen", onFullscreenChange, true);
+      info("Fullscreen event received");
+      resolve();
+    }
+    window.addEventListener("fullscreen", onFullscreenChange, true);
+  });
+}
diff --git a/browser/components/customizableui/test/browser_1087303_button_preferences.js b/browser/components/customizableui/test/browser_1087303_button_preferences.js
new file mode 100644
index 0000000000..7db48341cb
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1087303_button_preferences.js
@@ -0,0 +1,59 @@
+/* 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/. */
+/* eslint-disable mozilla/no-arbitrary-setTimeout */
+
+"use strict";
+
+var newTab = null;
+
+add_task(async function () {
+  info("Check preferences button existence and functionality");
+  CustomizableUI.addWidgetToArea(
+    "preferences-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await waitForOverflowButtonShown();
+
+  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);
+
+  let openedPage = gBrowser.currentURI.spec;
+  is(openedPage, "about:preferences", "Preferences page was opened");
+});
+
+add_task(function asyncCleanup() {
+  if (gBrowser.tabs.length == 1) {
+    BrowserTestUtils.addTab(gBrowser, "about:blank");
+  }
+
+  gBrowser.removeTab(gBrowser.selectedTab);
+  info("Tabs were restored");
+});
+
+function waitForPageLoad(aTab) {
+  return new Promise((resolve, reject) => {
+    let timeoutId = setTimeout(() => {
+      aTab.linkedBrowser.removeEventListener("load", onTabLoad, true);
+      reject("Page didn't load within " + 20000 + "ms");
+    }, 20000);
+
+    async function onTabLoad(event) {
+      clearTimeout(timeoutId);
+      aTab.linkedBrowser.removeEventListener("load", onTabLoad, true);
+      info("Tab event received: load");
+      resolve();
+    }
+
+    aTab.linkedBrowser.addEventListener("load", onTabLoad, true, true);
+  });
+}
diff --git a/browser/components/customizableui/test/browser_1089591_still_customizable_after_reset.js b/browser/components/customizableui/test/browser_1089591_still_customizable_after_reset.js
new file mode 100644
index 0000000000..b0bbbd726c
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1089591_still_customizable_after_reset.js
@@ -0,0 +1,24 @@
+"use strict";
+
+// Dragging the elements again after a reset should work
+add_task(async function () {
+  await startCustomizing();
+  let historyButton = document.getElementById("wrapper-history-panelmenu");
+  let devButton = document.getElementById("wrapper-developer-button");
+
+  ok(historyButton && devButton, "Draggable elements should exist");
+  simulateItemDrag(historyButton, devButton);
+  await gCustomizeMode.reset();
+  ok(CustomizableUI.inDefaultState, "Should be back in default state");
+
+  historyButton = document.getElementById("wrapper-history-panelmenu");
+  devButton = document.getElementById("wrapper-developer-button");
+  ok(historyButton && devButton, "Draggable elements should exist");
+  simulateItemDrag(historyButton, devButton);
+
+  await endCustomizing();
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_1096763_seen_widgets_post_reset.js b/browser/components/customizableui/test/browser_1096763_seen_widgets_post_reset.js
new file mode 100644
index 0000000000..74854f499c
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1096763_seen_widgets_post_reset.js
@@ -0,0 +1,41 @@
+"use strict";
+
+const BUTTONID = "test-seenwidget-post-reset";
+
+add_task(async function () {
+  CustomizableUI.createWidget({
+    id: BUTTONID,
+    label: "Test widget seen post reset",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  });
+
+  const kPrefCustomizationState = "browser.uiCustomization.state";
+  ok(
+    CustomizableUI.getTestOnlyInternalProp("gSeenWidgets").has(BUTTONID),
+    "Widget should be seen after createWidget is called."
+  );
+  CustomizableUI.reset();
+  ok(
+    CustomizableUI.getTestOnlyInternalProp("gSeenWidgets").has(BUTTONID),
+    "Widget should still be seen after reset."
+  );
+  CustomizableUI.addWidgetToArea(BUTTONID, CustomizableUI.AREA_NAVBAR);
+  gCustomizeMode.removeFromArea(document.getElementById(BUTTONID));
+  let hasUserValue = Services.prefs.prefHasUserValue(kPrefCustomizationState);
+  ok(hasUserValue, "Pref should be set right now.");
+  if (hasUserValue) {
+    let seenArray = JSON.parse(
+      Services.prefs.getCharPref(kPrefCustomizationState)
+    ).seen;
+    isnot(
+      seenArray.indexOf(BUTTONID),
+      -1,
+      "Widget should be in saved 'seen' list."
+    );
+  }
+});
+
+registerCleanupFunction(function () {
+  CustomizableUI.destroyWidget(BUTTONID);
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_1161838_inserted_new_default_buttons.js b/browser/components/customizableui/test/browser_1161838_inserted_new_default_buttons.js
new file mode 100644
index 0000000000..b9501e94f8
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1161838_inserted_new_default_buttons.js
@@ -0,0 +1,109 @@
+"use strict";
+
+// NB: This uses some ugly hacks to get into the CUI module from elsewhere...
+// don't try this at home, kids.
+function test() {
+  // Customize something to make sure stuff changed:
+  CustomizableUI.addWidgetToArea(
+    "save-page-button",
+    CustomizableUI.AREA_NAVBAR
+  );
+
+  let gFuturePlacements =
+    CustomizableUI.getTestOnlyInternalProp("gFuturePlacements");
+  is(
+    gFuturePlacements.size,
+    0,
+    "All future placements should be dealt with by now."
+  );
+
+  let CustomizableUIInternal = CustomizableUI.getTestOnlyInternalProp(
+    "CustomizableUIInternal"
+  );
+
+  // Force us to have a saved state:
+  CustomizableUIInternal.saveState();
+  CustomizableUIInternal.loadSavedState();
+
+  CustomizableUIInternal._updateForNewVersion();
+  is(gFuturePlacements.size, 0, "No change to future placements initially.");
+
+  // Add our widget to the defaults:
+  let testWidgetNew = {
+    id: "test-messing-with-default-placements-new-pref",
+    label: "Test messing with default placements - pref-based",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+    introducedInVersion: "pref",
+  };
+
+  let normalizedWidget = CustomizableUIInternal.normalizeWidget(
+    testWidgetNew,
+    CustomizableUI.SOURCE_BUILTIN
+  );
+  ok(normalizedWidget, "Widget should be normalizable");
+  if (!normalizedWidget) {
+    return;
+  }
+  let gPalette = CustomizableUI.getTestOnlyInternalProp("gPalette");
+  gPalette.set(testWidgetNew.id, normalizedWidget);
+
+  // Now adjust default placements for area:
+  let navbarArea = CustomizableUI.getTestOnlyInternalProp("gAreas").get(
+    CustomizableUI.AREA_NAVBAR
+  );
+  let navbarPlacements = navbarArea.get("defaultPlacements");
+  navbarPlacements.splice(
+    navbarPlacements.indexOf("bookmarks-menu-button") + 1,
+    0,
+    testWidgetNew.id
+  );
+
+  let savedPlacements =
+    CustomizableUI.getTestOnlyInternalProp("gSavedState").placements[
+      CustomizableUI.AREA_NAVBAR
+    ];
+  // Then call the re-init routine so we re-add the builtin widgets
+  CustomizableUIInternal._updateForNewVersion();
+  is(gFuturePlacements.size, 1, "Should have 1 more future placement");
+  let futureNavbarPlacements = gFuturePlacements.get(
+    CustomizableUI.AREA_NAVBAR
+  );
+  ok(futureNavbarPlacements, "Should have placements for nav-bar");
+  if (futureNavbarPlacements) {
+    ok(
+      futureNavbarPlacements.has(testWidgetNew.id),
+      "widget should be in future placements"
+    );
+  }
+  CustomizableUIInternal._placeNewDefaultWidgetsInArea(
+    CustomizableUI.AREA_NAVBAR
+  );
+
+  let indexInSavedPlacements = savedPlacements.indexOf(testWidgetNew.id);
+  info("Saved placements: " + savedPlacements.join(", "));
+  isnot(indexInSavedPlacements, -1, "Widget should have been inserted");
+  is(
+    indexInSavedPlacements,
+    savedPlacements.indexOf("bookmarks-menu-button") + 1,
+    "Widget should be in the right place."
+  );
+
+  if (futureNavbarPlacements) {
+    ok(
+      !futureNavbarPlacements.has(testWidgetNew.id),
+      "widget should be out of future placements"
+    );
+  }
+
+  if (indexInSavedPlacements != -1) {
+    savedPlacements.splice(indexInSavedPlacements, 1);
+  }
+
+  gFuturePlacements.delete(CustomizableUI.AREA_NAVBAR);
+  let indexInDefaultPlacements = navbarPlacements.indexOf(testWidgetNew.id);
+  if (indexInDefaultPlacements != -1) {
+    navbarPlacements.splice(indexInDefaultPlacements, 1);
+  }
+  gPalette.delete(testWidgetNew.id);
+  CustomizableUI.reset();
+}
diff --git a/browser/components/customizableui/test/browser_1484275_PanelMultiView_toggle_with_other_popup.js b/browser/components/customizableui/test/browser_1484275_PanelMultiView_toggle_with_other_popup.js
new file mode 100644
index 0000000000..89b86dba20
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1484275_PanelMultiView_toggle_with_other_popup.js
@@ -0,0 +1,69 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_URL = "data:text/html,<html><body></body></html>";
+
+/**
+ * Test steps that may lead to the panel being stuck on Windows (bug 1484275).
+ */
+add_task(async function test_PanelMultiView_toggle_with_other_popup() {
+  // For proper cleanup, create a bookmark that we will remove later.
+  let bookmark = await PlacesUtils.bookmarks.insert({
+    parentGuid: PlacesUtils.bookmarks.unfiledGuid,
+    url: TEST_URL,
+  });
+  registerCleanupFunction(() => PlacesUtils.bookmarks.remove(bookmark));
+
+  await BrowserTestUtils.withNewTab(
+    {
+      gBrowser,
+      url: TEST_URL,
+    },
+    async function (browser) {
+      // 1. Open the main menu.
+      await gCUITestUtils.openMainMenu();
+
+      // 2. Open another popup not managed by PanelMultiView.
+      StarUI._createPanelIfNeeded();
+      let bookmarkPanel = document.getElementById("editBookmarkPanel");
+      let shown = BrowserTestUtils.waitForEvent(bookmarkPanel, "popupshown");
+      let hidden = BrowserTestUtils.waitForEvent(bookmarkPanel, "popuphidden");
+      EventUtils.synthesizeKey("D", { accelKey: true });
+      await shown;
+
+      // 3. Click the button to which the main menu is anchored. We need a native
+      // mouse event to simulate the exact platform behavior with popups.
+      let clickFn = () =>
+        EventUtils.promiseNativeMouseEventAndWaitForEvent({
+          type: "click",
+          target: document.getElementById("PanelUI-button"),
+          atCenter: true,
+          eventTypeToWait: "mouseup",
+        });
+
+      // On Windows and macOS, the operation will close both popups.
+      if (AppConstants.platform == "win" || AppConstants.platform == "macosx") {
+        await gCUITestUtils.hidePanelMultiView(PanelUI.panel, clickFn);
+        await new Promise(resolve => executeSoon(resolve));
+
+        // 4. Test that the popup can be opened again after it's been closed.
+        await gCUITestUtils.openMainMenu();
+        Assert.equal(PanelUI.panel.state, "open");
+      } else {
+        // On other platforms, the operation will close both popups and reopen the
+        // main menu immediately, so we wait for the reopen to occur.
+        shown = BrowserTestUtils.waitForEvent(PanelUI.mainView, "ViewShown");
+        clickFn();
+        await shown;
+      }
+
+      await gCUITestUtils.hideMainMenu();
+
+      // Make sure the events for the bookmarks panel have also been processed
+      // before closing the tab and removing the bookmark.
+      await hidden;
+    }
+  );
+});
diff --git a/browser/components/customizableui/test/browser_1701883_restore_defaults_pocket_pref.js b/browser/components/customizableui/test/browser_1701883_restore_defaults_pocket_pref.js
new file mode 100644
index 0000000000..a2085958fd
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1701883_restore_defaults_pocket_pref.js
@@ -0,0 +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";
+
+// Turning off Pocket pref should still be considered default state.
+add_task(async function () {
+  ok(CustomizableUI.inDefaultState, "Default state to begin");
+
+  Assert.ok(
+    Services.prefs.getBoolPref("extensions.pocket.enabled"),
+    "Pocket feature is enabled by default"
+  );
+
+  Services.prefs.setBoolPref("extensions.pocket.enabled", false);
+
+  ok(CustomizableUI.inDefaultState, "Should still be default state");
+  await resetCustomization();
+
+  Assert.ok(
+    !Services.prefs.getBoolPref("extensions.pocket.enabled"),
+    "Pocket feature is still off"
+  );
+  ok(CustomizableUI.inDefaultState, "Should still be default state");
+
+  Services.prefs.setBoolPref("extensions.pocket.enabled", true);
+});
diff --git a/browser/components/customizableui/test/browser_1702200_PanelMultiView_header_separator.js b/browser/components/customizableui/test/browser_1702200_PanelMultiView_header_separator.js
new file mode 100644
index 0000000000..471d33c37a
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1702200_PanelMultiView_header_separator.js
@@ -0,0 +1,76 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Tests whether the separator insertion works correctly for the
+ * special case where we remove the content except the header itself
+ * before showing the panel view.
+ */
+
+const TEST_SITE = "http://127.0.0.1";
+const RECENTLY_CLOSED_TABS_PANEL_ID = "appMenu-library-recentlyClosedTabs";
+const RECENTLY_CLOSED_TABS_ITEM_ID = "appMenuRecentlyClosedTabs";
+
+function assertHeaderSeparator() {
+  let header = document.querySelector(
+    `#${RECENTLY_CLOSED_TABS_PANEL_ID} .panel-header`
+  );
+  Assert.equal(
+    header.nextSibling.tagName,
+    "toolbarseparator",
+    "toolbarseparator should be shown below header"
+  );
+}
+
+/**
+ * Open and close a tab so we can access the "Recently
+ * closed tabs" panel
+ */
+add_task(async function test_setup() {
+  let tab = BrowserTestUtils.addTab(gBrowser, TEST_SITE);
+  gBrowser.selectedTab = tab;
+
+  let browser = gBrowser.getBrowserForTab(tab);
+  await BrowserTestUtils.browserLoaded(browser, false, null, true);
+  await BrowserTestUtils.removeTab(tab);
+});
+
+/**
+ * Tests whether the toolbarseparator is shown correctly
+ * after re-entering same sub view, see bug 1702200
+ *
+ * - App Menu
+ *   - History
+ *     - Recently closed tabs
+ */
+add_task(async function test_header_toolbarseparator() {
+  await gCUITestUtils.openMainMenu();
+
+  let historyView = PanelMultiView.getViewNode(document, "PanelUI-history");
+  document.getElementById("appMenu-history-button").click();
+  await BrowserTestUtils.waitForEvent(historyView, "ViewShown");
+
+  // Open Recently Closed Tabs and make sure there is a header separator
+  let closedTabsView = PanelMultiView.getViewNode(
+    document,
+    RECENTLY_CLOSED_TABS_PANEL_ID
+  );
+  Assert.ok(!document.getElementById(RECENTLY_CLOSED_TABS_ITEM_ID).disabled);
+  document.getElementById(RECENTLY_CLOSED_TABS_ITEM_ID).click();
+  await BrowserTestUtils.waitForEvent(closedTabsView, "ViewShown");
+  assertHeaderSeparator();
+
+  // Go back and re-open the same view, header separator should be
+  // re-added as well
+  document
+    .querySelector(`#${RECENTLY_CLOSED_TABS_PANEL_ID} .subviewbutton-back`)
+    .click();
+  await BrowserTestUtils.waitForEvent(historyView, "ViewShown");
+  document.getElementById(RECENTLY_CLOSED_TABS_ITEM_ID).click();
+  await BrowserTestUtils.waitForEvent(closedTabsView, "ViewShown");
+  assertHeaderSeparator();
+
+  await gCUITestUtils.hideMainMenu();
+});
diff --git a/browser/components/customizableui/test/browser_1795260_searchbar_overflow_toolbar.js b/browser/components/customizableui/test/browser_1795260_searchbar_overflow_toolbar.js
new file mode 100644
index 0000000000..2191b153cb
--- /dev/null
+++ b/browser/components/customizableui/test/browser_1795260_searchbar_overflow_toolbar.js
@@ -0,0 +1,29 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const WIDGET_ID = "search-container";
+
+registerCleanupFunction(() => {
+  CustomizableUI.reset();
+  Services.prefs.clearUserPref("browser.search.widget.inNavBar");
+});
+
+add_task(async function test_syncPreferenceWithWidget() {
+  // Move the searchbar to the nav bar.
+  CustomizableUI.addWidgetToArea(WIDGET_ID, CustomizableUI.AREA_NAVBAR);
+
+  let container = document.getElementById(WIDGET_ID);
+  // Set a disproportionately large width, which could be from a saved bigger
+  // window, or what not.
+  let width = window.innerWidth * 2;
+  container.setAttribute("width", width);
+  container.style.width = `${width}px`;
+
+  // Stuff shouldn't overflow.
+  ok(
+    container.getBoundingClientRect().width < window.innerWidth,
+    "Searchbar shouldn't overflow"
+  );
+});
diff --git a/browser/components/customizableui/test/browser_694291_searchbar_preference.js b/browser/components/customizableui/test/browser_694291_searchbar_preference.js
new file mode 100644
index 0000000000..f65d8f0adc
--- /dev/null
+++ b/browser/components/customizableui/test/browser_694291_searchbar_preference.js
@@ -0,0 +1,48 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const WIDGET_ID = "search-container";
+const PREF_NAME = "browser.search.widget.inNavBar";
+
+function checkDefaults() {
+  ok(!Services.prefs.getBoolPref(PREF_NAME));
+  is(CustomizableUI.getPlacementOfWidget(WIDGET_ID), null);
+}
+
+add_task(async function test_defaults() {
+  // Verify the default state before the first test.
+  checkDefaults();
+});
+
+add_task(async function test_syncPreferenceWithWidget() {
+  // Moving the widget to any position in the navigation toolbar should turn the
+  // preference to true.
+  CustomizableUI.addWidgetToArea(WIDGET_ID, CustomizableUI.AREA_NAVBAR);
+  ok(Services.prefs.getBoolPref(PREF_NAME));
+
+  // Moving the widget to any position outside of the navigation toolbar should
+  // turn the preference back to false.
+  CustomizableUI.addWidgetToArea(
+    WIDGET_ID,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  ok(!Services.prefs.getBoolPref(PREF_NAME));
+});
+
+add_task(async function test_syncWidgetWithPreference() {
+  // setting the preference should move the widget to the navigation toolbar and
+  // place it right after the location bar.
+  Services.prefs.setBoolPref(PREF_NAME, true);
+  let placement = CustomizableUI.getPlacementOfWidget(WIDGET_ID);
+  is(placement.area, CustomizableUI.AREA_NAVBAR);
+  is(
+    placement.position,
+    CustomizableUI.getPlacementOfWidget("urlbar-container").position + 1
+  );
+
+  // This should move the widget back to the customization palette.
+  Services.prefs.setBoolPref(PREF_NAME, false);
+  checkDefaults();
+});
diff --git a/browser/components/customizableui/test/browser_873501_handle_specials.js b/browser/components/customizableui/test/browser_873501_handle_specials.js
new file mode 100644
index 0000000000..1711aee392
--- /dev/null
+++ b/browser/components/customizableui/test/browser_873501_handle_specials.js
@@ -0,0 +1,89 @@
+/* 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";
+
+const kToolbarName = "test-specials-toolbar";
+
+registerCleanupFunction(removeCustomToolbars);
+
+// Add a toolbar with two springs and the downloads button.
+add_task(async function addToolbarWith2SpringsAndDownloadsButton() {
+  // Create the toolbar with a single spring:
+  createToolbarWithPlacements(kToolbarName, ["spring"]);
+  ok(document.getElementById(kToolbarName), "Toolbar should be created.");
+
+  // Check it's there with a generated ID:
+  assertAreaPlacements(kToolbarName, [/customizableui-special-spring\d+/]);
+  let [springId] = getAreaWidgetIds(kToolbarName);
+
+  // Add a second spring, check if that's there and doesn't share IDs
+  CustomizableUI.addWidgetToArea("spring", kToolbarName);
+  assertAreaPlacements(kToolbarName, [
+    springId,
+    /customizableui-special-spring\d+/,
+  ]);
+  let [, spring2Id] = getAreaWidgetIds(kToolbarName);
+
+  isnot(springId, spring2Id, "Springs shouldn't have identical IDs.");
+
+  // Try moving the downloads button to this new toolbar, between the two springs:
+  CustomizableUI.addWidgetToArea("downloads-button", kToolbarName, 1);
+  assertAreaPlacements(kToolbarName, [springId, "downloads-button", spring2Id]);
+  await removeCustomToolbars();
+});
+
+// Add separators around the downloads button.
+add_task(async function addSeparatorsAroundDownloadsButton() {
+  createToolbarWithPlacements(kToolbarName, ["separator"]);
+  ok(document.getElementById(kToolbarName), "Toolbar should be created.");
+
+  // Check it's there with a generated ID:
+  assertAreaPlacements(kToolbarName, [/customizableui-special-separator\d+/]);
+  let [separatorId] = getAreaWidgetIds(kToolbarName);
+
+  CustomizableUI.addWidgetToArea("separator", kToolbarName);
+  assertAreaPlacements(kToolbarName, [
+    separatorId,
+    /customizableui-special-separator\d+/,
+  ]);
+  let [, separator2Id] = getAreaWidgetIds(kToolbarName);
+
+  isnot(separatorId, separator2Id, "Separator ids shouldn't be equal.");
+
+  CustomizableUI.addWidgetToArea("downloads-button", kToolbarName, 1);
+  assertAreaPlacements(kToolbarName, [
+    separatorId,
+    "downloads-button",
+    separator2Id,
+  ]);
+  await removeCustomToolbars();
+});
+
+// Add spacers around the downloads button.
+add_task(async function addSpacersAroundDownloadsButton() {
+  createToolbarWithPlacements(kToolbarName, ["spacer"]);
+  ok(document.getElementById(kToolbarName), "Toolbar should be created.");
+
+  // Check it's there with a generated ID:
+  assertAreaPlacements(kToolbarName, [/customizableui-special-spacer\d+/]);
+  let [spacerId] = getAreaWidgetIds(kToolbarName);
+
+  CustomizableUI.addWidgetToArea("spacer", kToolbarName);
+  assertAreaPlacements(kToolbarName, [
+    spacerId,
+    /customizableui-special-spacer\d+/,
+  ]);
+  let [, spacer2Id] = getAreaWidgetIds(kToolbarName);
+
+  isnot(spacerId, spacer2Id, "Spacer ids shouldn't be equal.");
+
+  CustomizableUI.addWidgetToArea("downloads-button", kToolbarName, 1);
+  assertAreaPlacements(kToolbarName, [spacerId, "downloads-button", spacer2Id]);
+  await removeCustomToolbars();
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_876926_customize_mode_wrapping.js b/browser/components/customizableui/test/browser_876926_customize_mode_wrapping.js
new file mode 100644
index 0000000000..cf73326e53
--- /dev/null
+++ b/browser/components/customizableui/test/browser_876926_customize_mode_wrapping.js
@@ -0,0 +1,295 @@
+/* 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";
+
+const kXULWidgetId = "a-test-button"; // we'll create a button with this ID.
+const kAPIWidgetId = "save-page-button";
+const kPanel = CustomizableUI.AREA_FIXED_OVERFLOW_PANEL;
+const kToolbar = CustomizableUI.AREA_NAVBAR;
+const kVisiblePalette = "customization-palette";
+
+function checkWrapper(id) {
+  is(
+    document.querySelectorAll("#wrapper-" + id).length,
+    1,
+    "There should be exactly 1 wrapper for " +
+      id +
+      " in the customizing window."
+  );
+}
+
+async function ensureVisible(node) {
+  let isInPalette = node.parentNode.parentNode == gNavToolbox.palette;
+  if (isInPalette) {
+    node.scrollIntoView();
+  }
+  let dwu = window.windowUtils;
+  await BrowserTestUtils.waitForCondition(() => {
+    let nodeBounds = dwu.getBoundsWithoutFlushing(node);
+    if (isInPalette) {
+      let paletteBounds = dwu.getBoundsWithoutFlushing(gNavToolbox.palette);
+      if (
+        !(
+          nodeBounds.top >= paletteBounds.top &&
+          nodeBounds.bottom <= paletteBounds.bottom
+        )
+      ) {
+        return false;
+      }
+    }
+    return nodeBounds.height && nodeBounds.width;
+  });
+}
+
+var move = {
+  async drag(id, target) {
+    let targetNode = document.getElementById(target);
+    if (CustomizableUI.getCustomizationTarget(targetNode)) {
+      targetNode = CustomizableUI.getCustomizationTarget(targetNode);
+    }
+    let nodeToMove = document.getElementById(id);
+    await ensureVisible(nodeToMove);
+
+    simulateItemDrag(nodeToMove, targetNode, "end");
+  },
+  async dragToItem(id, target) {
+    let targetNode = document.getElementById(target);
+    if (CustomizableUI.getCustomizationTarget(targetNode)) {
+      targetNode = CustomizableUI.getCustomizationTarget(targetNode);
+    }
+    let items = targetNode.querySelectorAll("toolbarpaletteitem");
+    if (target == kPanel) {
+      targetNode = items[items.length - 1];
+    } else {
+      targetNode = items[0];
+    }
+    let nodeToMove = document.getElementById(id);
+    await ensureVisible(nodeToMove);
+    simulateItemDrag(nodeToMove, targetNode, "start");
+  },
+  API(id, target) {
+    if (target == kVisiblePalette) {
+      return CustomizableUI.removeWidgetFromArea(id);
+    }
+    return CustomizableUI.addWidgetToArea(id, target, null);
+  },
+};
+
+function isLast(containerId, defaultPlacements, id) {
+  assertAreaPlacements(containerId, defaultPlacements.concat([id]));
+  let thisTarget = CustomizableUI.getCustomizationTarget(
+    document.getElementById(containerId)
+  );
+  is(
+    thisTarget.lastElementChild.firstElementChild.id,
+    id,
+    "Widget " + id + " should be in " + containerId + " in customizing window."
+  );
+  let otherTarget = CustomizableUI.getCustomizationTarget(
+    otherWin.document.getElementById(containerId)
+  );
+  is(
+    otherTarget.lastElementChild.id,
+    id,
+    "Widget " + id + " should be in " + containerId + " in other window."
+  );
+}
+
+function getLastVisibleNodeInToolbar(containerId, win = window) {
+  let container = CustomizableUI.getCustomizationTarget(
+    win.document.getElementById(containerId)
+  );
+  let rv = container.lastElementChild;
+  while (rv?.hidden || rv?.firstElementChild?.hidden) {
+    rv = rv.previousElementSibling;
+  }
+  return rv;
+}
+
+function isLastVisibleInToolbar(containerId, defaultPlacements, id) {
+  let newPlacements;
+  for (let i = defaultPlacements.length - 1; i >= 0; i--) {
+    let el = document.getElementById(defaultPlacements[i]);
+    if (el && !el.hidden) {
+      newPlacements = [...defaultPlacements];
+      newPlacements.splice(i + 1, 0, id);
+      break;
+    }
+  }
+  if (!newPlacements) {
+    assertAreaPlacements(containerId, defaultPlacements.concat([id]));
+  } else {
+    assertAreaPlacements(containerId, newPlacements);
+  }
+  is(
+    getLastVisibleNodeInToolbar(containerId).firstElementChild.id,
+    id,
+    "Widget " + id + " should be in " + containerId + " in customizing window."
+  );
+  is(
+    getLastVisibleNodeInToolbar(containerId, otherWin).id,
+    id,
+    "Widget " + id + " should be in " + containerId + " in other window."
+  );
+}
+
+function isFirst(containerId, defaultPlacements, id) {
+  assertAreaPlacements(containerId, [id].concat(defaultPlacements));
+  let thisTarget = CustomizableUI.getCustomizationTarget(
+    document.getElementById(containerId)
+  );
+  is(
+    thisTarget.firstElementChild.firstElementChild.id,
+    id,
+    "Widget " + id + " should be in " + containerId + " in customizing window."
+  );
+  let otherTarget = CustomizableUI.getCustomizationTarget(
+    otherWin.document.getElementById(containerId)
+  );
+  is(
+    otherTarget.firstElementChild.id,
+    id,
+    "Widget " + id + " should be in " + containerId + " in other window."
+  );
+}
+
+async function checkToolbar(id, method) {
+  // Place at start of the toolbar:
+  let toolbarPlacements = getAreaWidgetIds(kToolbar);
+  await move[method](id, kToolbar);
+  if (method == "dragToItem") {
+    isFirst(kToolbar, toolbarPlacements, id);
+  } else if (method == "drag") {
+    isLastVisibleInToolbar(kToolbar, toolbarPlacements, id);
+  } else {
+    isLast(kToolbar, toolbarPlacements, id);
+  }
+  checkWrapper(id);
+}
+
+async function checkPanel(id, method) {
+  let panelPlacements = getAreaWidgetIds(kPanel);
+  await move[method](id, kPanel);
+  let children = document
+    .getElementById(kPanel)
+    .querySelectorAll("toolbarpaletteitem");
+  let otherChildren = otherWin.document.getElementById(kPanel).children;
+  let newPlacements = panelPlacements.concat([id]);
+  // Relative position of the new item from the end:
+  let position = -1;
+  // For the drag to item case, we drag to the last item, making the dragged item the
+  // penultimate item. We can't well use the first item because the panel has complicated
+  // rules about rearranging wide items (which, by default, the first two items are).
+  if (method == "dragToItem") {
+    newPlacements.pop();
+    newPlacements.splice(panelPlacements.length - 1, 0, id);
+    position = -2;
+  }
+  assertAreaPlacements(kPanel, newPlacements);
+  is(
+    children[children.length + position].firstElementChild.id,
+    id,
+    "Widget " + id + " should be in " + kPanel + " in customizing window."
+  );
+  is(
+    otherChildren[otherChildren.length + position].id,
+    id,
+    "Widget " + id + " should be in " + kPanel + " in other window."
+  );
+  checkWrapper(id);
+}
+
+async function checkPalette(id, method) {
+  // Move back to palette:
+  await move[method](id, kVisiblePalette);
+  ok(CustomizableUI.inDefaultState, "Should end in default state");
+  let visibleChildren = gCustomizeMode.visiblePalette.children;
+  let expectedChild =
+    method == "dragToItem"
+      ? visibleChildren[0]
+      : visibleChildren[visibleChildren.length - 1];
+  // Items dragged to the end of the palette should be the final item. That they're the penultimate
+  // item when dragged is tracked in bug 1395950. Once that's fixed, this hack can be removed.
+  if (method == "drag") {
+    expectedChild = expectedChild.previousElementSibling;
+  }
+  is(
+    expectedChild.firstElementChild.id,
+    id,
+    "Widget " +
+      id +
+      " was moved using " +
+      method +
+      " and should now be wrapped in palette in customizing window."
+  );
+  if (id == kXULWidgetId) {
+    ok(
+      otherWin.gNavToolbox.palette.querySelector("#" + id),
+      "Widget " + id + " should be in invisible palette in other window."
+    );
+  }
+  checkWrapper(id);
+}
+
+// This test needs a XUL button that's in the palette by default. No such
+// button currently exists, so we create a simple one.
+function createXULButtonForWindow(win) {
+  createDummyXULButton(kXULWidgetId, "test-button", win);
+}
+
+function removeXULButtonForWindow(win) {
+  win.gNavToolbox.palette.querySelector(`#${kXULWidgetId}`).remove();
+}
+
+var otherWin;
+
+// Moving widgets in two windows, one with customize mode and one without, should work.
+add_task(async function MoveWidgetsInTwoWindows() {
+  CustomizableUI.createWidget({
+    id: "cui-mode-wrapping-some-panel-item",
+    label: "Test panel wrapping",
+  });
+  await startCustomizing();
+  otherWin = await openAndLoadWindow(null, true);
+  await otherWin.PanelUI.ensureReady();
+  // Create the XUL button to use in the test in both windows.
+  createXULButtonForWindow(window);
+  createXULButtonForWindow(otherWin);
+  ok(CustomizableUI.inDefaultState, "Should start in default state");
+
+  for (let widgetId of [kXULWidgetId, kAPIWidgetId]) {
+    for (let method of ["API", "drag", "dragToItem"]) {
+      info("Moving widget " + widgetId + " using " + method);
+      await checkToolbar(widgetId, method);
+      // We add an item to the panel because otherwise we can't test dragging
+      // to items that are already there. We remove it because
+      // 'checkPalette' checks that we leave the browser in the default state.
+      CustomizableUI.addWidgetToArea(
+        "cui-mode-wrapping-some-panel-item",
+        CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+      );
+      await checkPanel(widgetId, method);
+      CustomizableUI.removeWidgetFromArea("cui-mode-wrapping-some-panel-item");
+      await checkPalette(widgetId, method);
+      CustomizableUI.addWidgetToArea(
+        "cui-mode-wrapping-some-panel-item",
+        CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+      );
+      await checkPanel(widgetId, method);
+      await checkToolbar(widgetId, method);
+      CustomizableUI.removeWidgetFromArea("cui-mode-wrapping-some-panel-item");
+      await checkPalette(widgetId, method);
+    }
+  }
+  await promiseWindowClosed(otherWin);
+  otherWin = null;
+  await endCustomizing();
+  removeXULButtonForWindow(window);
+});
+
+add_task(async function asyncCleanup() {
+  CustomizableUI.destroyWidget("cui-mode-wrapping-some-panel-item");
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_876944_customize_mode_create_destroy.js b/browser/components/customizableui/test/browser_876944_customize_mode_create_destroy.js
new file mode 100644
index 0000000000..33eccccbbf
--- /dev/null
+++ b/browser/components/customizableui/test/browser_876944_customize_mode_create_destroy.js
@@ -0,0 +1,44 @@
+/* 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";
+
+const kTestWidget1 = "test-customize-mode-create-destroy1";
+
+// Creating and destroying a widget should correctly wrap/unwrap stuff
+add_task(async function testWrapUnwrap() {
+  await startCustomizing();
+  CustomizableUI.createWidget({
+    id: kTestWidget1,
+    label: "Pretty label",
+    tooltiptext: "Pretty tooltip",
+  });
+  let elem = document.getElementById(kTestWidget1);
+  let wrapper = document.getElementById("wrapper-" + kTestWidget1);
+  ok(elem, "There should be an item");
+  ok(wrapper, "There should be a wrapper");
+  is(
+    wrapper.firstElementChild.id,
+    kTestWidget1,
+    "Wrapper should have test widget"
+  );
+  is(
+    wrapper.parentNode.id,
+    "customization-palette",
+    "Wrapper should be in palette"
+  );
+  CustomizableUI.destroyWidget(kTestWidget1);
+  wrapper = document.getElementById("wrapper-" + kTestWidget1);
+  ok(!wrapper, "There should be a wrapper");
+  let item = document.getElementById(kTestWidget1);
+  ok(!item, "There should no longer be an item");
+});
+
+add_task(async function asyncCleanup() {
+  await endCustomizing();
+  try {
+    CustomizableUI.destroyWidget(kTestWidget1);
+  } catch (ex) {}
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_877006_missing_view.js b/browser/components/customizableui/test/browser_877006_missing_view.js
new file mode 100644
index 0000000000..c01d2f7b35
--- /dev/null
+++ b/browser/components/customizableui/test/browser_877006_missing_view.js
@@ -0,0 +1,46 @@
+/* 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";
+
+// Should be able to add broken view widget
+add_task(function testAddbrokenViewWidget() {
+  const kWidgetId = "test-877006-broken-widget";
+  let widgetSpec = {
+    id: kWidgetId,
+    type: "view",
+    viewId: "idontexist",
+    /* Empty handler so we try to attach it maybe? */
+    onViewShowing() {},
+  };
+
+  let noError = true;
+  try {
+    CustomizableUI.createWidget(widgetSpec);
+    CustomizableUI.addWidgetToArea(kWidgetId, CustomizableUI.AREA_NAVBAR);
+  } catch (ex) {
+    console.error(ex);
+    noError = false;
+  }
+  ok(
+    noError,
+    "Should not throw an exception trying to add a broken view widget."
+  );
+
+  noError = true;
+  try {
+    CustomizableUI.destroyWidget(kWidgetId);
+  } catch (ex) {
+    console.error(ex);
+    noError = false;
+  }
+  ok(
+    noError,
+    "Should not throw an exception trying to remove the broken view widget."
+  );
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_877178_unregisterArea.js b/browser/components/customizableui/test/browser_877178_unregisterArea.js
new file mode 100644
index 0000000000..7b171462ff
--- /dev/null
+++ b/browser/components/customizableui/test/browser_877178_unregisterArea.js
@@ -0,0 +1,70 @@
+/* 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";
+
+registerCleanupFunction(removeCustomToolbars);
+
+// Sanity checks
+add_task(function sanityChecks() {
+  SimpleTest.doesThrow(
+    () => CustomizableUI.registerArea("@foo"),
+    "Registering areas with an invalid ID should throw."
+  );
+
+  SimpleTest.doesThrow(
+    () => CustomizableUI.registerArea([]),
+    "Registering areas with an invalid ID should throw."
+  );
+
+  SimpleTest.doesThrow(
+    () => CustomizableUI.unregisterArea("@foo"),
+    "Unregistering areas with an invalid ID should throw."
+  );
+
+  SimpleTest.doesThrow(
+    () => CustomizableUI.unregisterArea([]),
+    "Unregistering areas with an invalid ID should throw."
+  );
+
+  SimpleTest.doesThrow(
+    () => CustomizableUI.unregisterArea("unknown"),
+    "Unregistering an area that's not registered should throw."
+  );
+});
+
+// Check areas are loaded with their default placements.
+add_task(function checkLoadedAres() {
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state."
+  );
+});
+
+// Check registering and unregistering a new area.
+add_task(function checkRegisteringAndUnregistering() {
+  const kToolbarId = "test-registration-toolbar";
+  const kButtonId = "test-registration-button";
+  createDummyXULButton(kButtonId);
+  createToolbarWithPlacements(kToolbarId, ["spring", kButtonId, "spring"]);
+  assertAreaPlacements(kToolbarId, [
+    /customizableui-special-spring\d+/,
+    kButtonId,
+    /customizableui-special-spring\d+/,
+  ]);
+  ok(
+    !CustomizableUI.inDefaultState,
+    "With a new toolbar it is no longer in a default state."
+  );
+  removeCustomToolbars(); // Will call unregisterArea for us
+  ok(
+    CustomizableUI.inDefaultState,
+    "When the toolbar is unregistered, " +
+      "everything will return to the default state."
+  );
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_877447_skip_missing_ids.js b/browser/components/customizableui/test/browser_877447_skip_missing_ids.js
new file mode 100644
index 0000000000..83e7edbba3
--- /dev/null
+++ b/browser/components/customizableui/test/browser_877447_skip_missing_ids.js
@@ -0,0 +1,35 @@
+/* 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";
+
+registerCleanupFunction(removeCustomToolbars);
+
+add_task(function skipMissingIDS() {
+  const kButtonId = "look-at-me-disappear-button";
+  CustomizableUI.reset();
+  ok(CustomizableUI.inDefaultState, "Should be in the default state.");
+  let btn = createDummyXULButton(kButtonId, "Gone!");
+  CustomizableUI.addWidgetToArea(kButtonId, CustomizableUI.AREA_NAVBAR);
+  ok(
+    !CustomizableUI.inDefaultState,
+    "Should no longer be in the default state."
+  );
+  is(
+    btn.parentNode.parentNode.id,
+    CustomizableUI.AREA_NAVBAR,
+    "Button should be in navbar"
+  );
+  btn.remove();
+  is(btn.parentNode, null, "Button is no longer in the navbar");
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be back in the default state, " +
+      "despite unknown button ID in placements."
+  );
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_878452_drag_to_panel.js b/browser/components/customizableui/test/browser_878452_drag_to_panel.js
new file mode 100644
index 0000000000..284583c853
--- /dev/null
+++ b/browser/components/customizableui/test/browser_878452_drag_to_panel.js
@@ -0,0 +1,90 @@
+/* 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";
+CustomizableUI.createWidget({
+  id: "cui-panel-item-to-drag-to",
+  defaultArea: CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+  label: "Item in panel to drag to",
+});
+
+// Dragging an item from the palette to another button in the panel should work.
+add_task(async function () {
+  await startCustomizing();
+  let btn = document.getElementById("new-window-button");
+  let placements = getAreaWidgetIds(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+
+  let lastButtonIndex = placements.length - 1;
+  let lastButton = placements[lastButtonIndex];
+  let placementsAfterInsert = placements
+    .slice(0, lastButtonIndex)
+    .concat(["new-window-button", lastButton]);
+  let lastButtonNode = document.getElementById(lastButton);
+  simulateItemDrag(btn, lastButtonNode, "start");
+  assertAreaPlacements(
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+    placementsAfterInsert
+  );
+  ok(!CustomizableUI.inDefaultState, "Should no longer be in default state.");
+  let palette = document.getElementById("customization-palette");
+  simulateItemDrag(btn, palette);
+  CustomizableUI.removeWidgetFromArea("cui-panel-item-to-drag-to");
+  ok(CustomizableUI.inDefaultState, "Should be in default state again.");
+  await endCustomizing();
+});
+
+// Dragging an item from the palette to the panel itself should also work.
+add_task(async function () {
+  CustomizableUI.addWidgetToArea(
+    "cui-panel-item-to-drag-to",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  await startCustomizing();
+  let btn = document.getElementById("new-window-button");
+  let panel = document.getElementById(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  let placements = getAreaWidgetIds(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+
+  let placementsAfterAppend = placements.concat(["new-window-button"]);
+  simulateItemDrag(btn, panel);
+  assertAreaPlacements(
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+    placementsAfterAppend
+  );
+  ok(!CustomizableUI.inDefaultState, "Should no longer be in default state.");
+  let palette = document.getElementById("customization-palette");
+  simulateItemDrag(btn, palette);
+  CustomizableUI.removeWidgetFromArea("cui-panel-item-to-drag-to");
+  ok(CustomizableUI.inDefaultState, "Should be in default state again.");
+  await endCustomizing();
+});
+
+// Dragging an item from the palette to an empty panel should also work.
+add_task(async function () {
+  let widgetIds = getAreaWidgetIds(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+  while (widgetIds.length) {
+    CustomizableUI.removeWidgetFromArea(widgetIds.shift());
+  }
+  await startCustomizing();
+  let btn = document.getElementById("new-window-button");
+  let panel = document.getElementById(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL);
+
+  assertAreaPlacements(panel.id, []);
+
+  let placementsAfterAppend = ["new-window-button"];
+  simulateItemDrag(btn, panel);
+  assertAreaPlacements(
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+    placementsAfterAppend
+  );
+  ok(!CustomizableUI.inDefaultState, "Should no longer be in default state.");
+  let palette = document.getElementById("customization-palette");
+  simulateItemDrag(btn, palette);
+  assertAreaPlacements(panel.id, []);
+  await endCustomizing();
+});
+
+registerCleanupFunction(async function asyncCleanup() {
+  CustomizableUI.destroyWidget("cui-panel-item-to-drag-to");
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_884402_customize_from_overflow.js b/browser/components/customizableui/test/browser_884402_customize_from_overflow.js
new file mode 100644
index 0000000000..583a03db2f
--- /dev/null
+++ b/browser/components/customizableui/test/browser_884402_customize_from_overflow.js
@@ -0,0 +1,117 @@
+"use strict";
+
+var overflowPanel = document.getElementById("widget-overflow");
+
+var originalWindowWidth;
+registerCleanupFunction(function () {
+  overflowPanel.removeAttribute("animate");
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  return TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+// Right-click on an item within the overflow panel should
+// show a context menu with options to move it.
+add_task(async function () {
+  overflowPanel.setAttribute("animate", "false");
+  let fxaButton = document.getElementById("fxa-toolbar-menu-button");
+  if (BrowserTestUtils.is_hidden(fxaButton)) {
+    // FxA button is likely hidden since the user is logged out.
+    let initialFxaStatus = document.documentElement.getAttribute("fxastatus");
+    document.documentElement.setAttribute("fxastatus", "signed_in");
+    registerCleanupFunction(() =>
+      document.documentElement.setAttribute("fxastatus", initialFxaStatus)
+    );
+    ok(BrowserTestUtils.is_visible(fxaButton), "FxA button is now visible");
+  }
+
+  originalWindowWidth = window.outerWidth;
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start with a non-overflowing toolbar."
+  );
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+
+  await TestUtils.waitForCondition(() => navbar.hasAttribute("overflowing"));
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+
+  let chevron = document.getElementById("nav-bar-overflow-button");
+  let shownPanelPromise = promisePanelElementShown(window, overflowPanel);
+  chevron.click();
+  await shownPanelPromise;
+
+  let contextMenu = document.getElementById(
+    "customizationPanelItemContextMenu"
+  );
+  let shownContextPromise = popupShown(contextMenu);
+  ok(fxaButton, "fxa-toolbar-menu-button was found");
+  is(
+    fxaButton.getAttribute("overflowedItem"),
+    "true",
+    "FxA button is overflowing"
+  );
+  EventUtils.synthesizeMouse(fxaButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownContextPromise;
+
+  is(
+    overflowPanel.state,
+    "open",
+    "The widget overflow panel should still be open."
+  );
+
+  let expectedEntries = [
+    [".customize-context-moveToPanel", true],
+    [".customize-context-removeFromPanel", true],
+    ["---"],
+    [".viewCustomizeToolbar", true],
+  ];
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  let hiddenPromise = promisePanelElementHidden(window, overflowPanel);
+  let moveToPanel = contextMenu.querySelector(".customize-context-moveToPanel");
+  if (moveToPanel) {
+    contextMenu.activateItem(moveToPanel);
+  } else {
+    contextMenu.hidePopup();
+  }
+  await hiddenContextPromise;
+  await hiddenPromise;
+
+  let fxaButtonPlacement = CustomizableUI.getPlacementOfWidget(
+    "fxa-toolbar-menu-button"
+  );
+  ok(fxaButtonPlacement, "FxA button should still have a placement");
+  is(
+    fxaButtonPlacement && fxaButtonPlacement.area,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+    "FxA button should be pinned now"
+  );
+  CustomizableUI.reset();
+
+  // In some cases, it can take a tick for the navbar to overflow again. Wait for it:
+  await TestUtils.waitForCondition(() =>
+    fxaButton.hasAttribute("overflowedItem")
+  );
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+
+  fxaButtonPlacement = CustomizableUI.getPlacementOfWidget(
+    "fxa-toolbar-menu-button"
+  );
+  ok(fxaButtonPlacement, "FxA button should still have a placement");
+  is(
+    fxaButtonPlacement && fxaButtonPlacement.area,
+    "nav-bar",
+    "FxA button should be back in the navbar now"
+  );
+
+  is(
+    fxaButton.getAttribute("overflowedItem"),
+    "true",
+    "FxA button should still be overflowed"
+  );
+});
diff --git a/browser/components/customizableui/test/browser_885052_customize_mode_observers_disabed.js b/browser/components/customizableui/test/browser_885052_customize_mode_observers_disabed.js
new file mode 100644
index 0000000000..346608dc99
--- /dev/null
+++ b/browser/components/customizableui/test/browser_885052_customize_mode_observers_disabed.js
@@ -0,0 +1,73 @@
+/* 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";
+
+function isFullscreenSizeMode() {
+  let sizemode = document.documentElement.getAttribute("sizemode");
+  return sizemode == "fullscreen";
+}
+
+// Observers should be disabled when in customization mode.
+add_task(async function () {
+  CustomizableUI.addWidgetToArea(
+    "fullscreen-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  // Show the panel so it isn't hidden and has bindings applied etc.:
+  await document.getElementById("nav-bar").overflowable.show();
+
+  // Hide it again.
+  document.getElementById("widget-overflow").hidePopup();
+
+  let fullscreenButton = document.getElementById("fullscreen-button");
+  ok(
+    !fullscreenButton.checked,
+    "Fullscreen button should not be checked when not in fullscreen."
+  );
+  ok(
+    !isFullscreenSizeMode(),
+    "Should not be in fullscreen sizemode before we enter fullscreen."
+  );
+
+  BrowserFullScreen();
+  await TestUtils.waitForCondition(() => isFullscreenSizeMode());
+  ok(
+    fullscreenButton.checked,
+    "Fullscreen button should be checked when in fullscreen."
+  );
+
+  await startCustomizing();
+
+  let fullscreenButtonWrapper = document.getElementById(
+    "wrapper-fullscreen-button"
+  );
+  ok(
+    fullscreenButtonWrapper.hasAttribute("itemobserves"),
+    "Observer should be moved to wrapper"
+  );
+  fullscreenButton = document.getElementById("fullscreen-button");
+  ok(
+    !fullscreenButton.hasAttribute("observes"),
+    "Observer should be removed from button"
+  );
+  ok(
+    !fullscreenButton.checked,
+    "Fullscreen button should no longer be checked during customization mode"
+  );
+
+  await endCustomizing();
+
+  BrowserFullScreen();
+  fullscreenButton = document.getElementById("fullscreen-button");
+  await TestUtils.waitForCondition(() => !isFullscreenSizeMode());
+  ok(
+    !fullscreenButton.checked,
+    "Fullscreen button should not be checked when not in fullscreen."
+  );
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_885530_showInPrivateBrowsing.js b/browser/components/customizableui/test/browser_885530_showInPrivateBrowsing.js
new file mode 100644
index 0000000000..e1f763e2eb
--- /dev/null
+++ b/browser/components/customizableui/test/browser_885530_showInPrivateBrowsing.js
@@ -0,0 +1,141 @@
+/* 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";
+
+const kWidgetId = "some-widget";
+
+function assertWidgetExists(aWindow, aExists) {
+  if (aExists) {
+    ok(
+      aWindow.document.getElementById(kWidgetId),
+      "Should have found test widget in the window"
+    );
+  } else {
+    is(
+      aWindow.document.getElementById(kWidgetId),
+      null,
+      "Should not have found test widget in the window"
+    );
+  }
+}
+
+// A widget that is created with showInPrivateBrowsing undefined should
+// have that value default to true.
+add_task(function () {
+  let wrapper = CustomizableUI.createWidget({
+    id: kWidgetId,
+  });
+  ok(
+    wrapper.showInPrivateBrowsing,
+    "showInPrivateBrowsing should have defaulted to true."
+  );
+  CustomizableUI.destroyWidget(kWidgetId);
+});
+
+// Add a widget via the API with showInPrivateBrowsing set to false
+// and ensure it does not appear in pre-existing or newly created
+// private windows.
+add_task(async function () {
+  let plain1 = await openAndLoadWindow();
+  let private1 = await openAndLoadWindow({ private: true });
+  CustomizableUI.createWidget({
+    id: kWidgetId,
+    removable: true,
+    showInPrivateBrowsing: false,
+  });
+  CustomizableUI.addWidgetToArea(kWidgetId, CustomizableUI.AREA_NAVBAR);
+  assertWidgetExists(plain1, true);
+  assertWidgetExists(private1, false);
+
+  // Now open up some new windows. The widget should exist in the new
+  // plain window, but not the new private window.
+  let plain2 = await openAndLoadWindow();
+  let private2 = await openAndLoadWindow({ private: true });
+  assertWidgetExists(plain2, true);
+  assertWidgetExists(private2, false);
+
+  // Try moving the widget around and make sure it doesn't get added
+  // to the private windows. We'll start by appending it to the tabstrip.
+  CustomizableUI.addWidgetToArea(kWidgetId, CustomizableUI.AREA_TABSTRIP);
+  assertWidgetExists(plain1, true);
+  assertWidgetExists(plain2, true);
+  assertWidgetExists(private1, false);
+  assertWidgetExists(private2, false);
+
+  // And then move it to the beginning of the tabstrip.
+  CustomizableUI.moveWidgetWithinArea(kWidgetId, 0);
+  assertWidgetExists(plain1, true);
+  assertWidgetExists(plain2, true);
+  assertWidgetExists(private1, false);
+  assertWidgetExists(private2, false);
+
+  CustomizableUI.removeWidgetFromArea("some-widget");
+  assertWidgetExists(plain1, false);
+  assertWidgetExists(plain2, false);
+  assertWidgetExists(private1, false);
+  assertWidgetExists(private2, false);
+
+  await Promise.all(
+    [plain1, plain2, private1, private2].map(promiseWindowClosed)
+  );
+
+  CustomizableUI.destroyWidget("some-widget");
+});
+
+// Add a widget via the API with showInPrivateBrowsing set to true,
+// and ensure that it appears in pre-existing or newly created
+// private browsing windows.
+add_task(async function () {
+  let plain1 = await openAndLoadWindow();
+  let private1 = await openAndLoadWindow({ private: true });
+
+  CustomizableUI.createWidget({
+    id: kWidgetId,
+    removable: true,
+    showInPrivateBrowsing: true,
+  });
+  CustomizableUI.addWidgetToArea(kWidgetId, CustomizableUI.AREA_NAVBAR);
+  assertWidgetExists(plain1, true);
+  assertWidgetExists(private1, true);
+
+  // Now open up some new windows. The widget should exist in the new
+  // plain window, but not the new private window.
+  let plain2 = await openAndLoadWindow();
+  let private2 = await openAndLoadWindow({ private: true });
+
+  assertWidgetExists(plain2, true);
+  assertWidgetExists(private2, true);
+
+  // Try moving the widget around and make sure it doesn't get added
+  // to the private windows. We'll start by appending it to the tabstrip.
+  CustomizableUI.addWidgetToArea(kWidgetId, CustomizableUI.AREA_TABSTRIP);
+  assertWidgetExists(plain1, true);
+  assertWidgetExists(plain2, true);
+  assertWidgetExists(private1, true);
+  assertWidgetExists(private2, true);
+
+  // And then move it to the beginning of the tabstrip.
+  CustomizableUI.moveWidgetWithinArea(kWidgetId, 0);
+  assertWidgetExists(plain1, true);
+  assertWidgetExists(plain2, true);
+  assertWidgetExists(private1, true);
+  assertWidgetExists(private2, true);
+
+  CustomizableUI.removeWidgetFromArea("some-widget");
+  assertWidgetExists(plain1, false);
+  assertWidgetExists(plain2, false);
+  assertWidgetExists(private1, false);
+  assertWidgetExists(private2, false);
+
+  await Promise.all(
+    [plain1, plain2, private1, private2].map(promiseWindowClosed)
+  );
+
+  CustomizableUI.destroyWidget("some-widget");
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_886323_buildArea_removable_nodes.js b/browser/components/customizableui/test/browser_886323_buildArea_removable_nodes.js
new file mode 100644
index 0000000000..99968a8266
--- /dev/null
+++ b/browser/components/customizableui/test/browser_886323_buildArea_removable_nodes.js
@@ -0,0 +1,58 @@
+/* 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";
+
+const kButtonId = "test-886323-removable-moved-node";
+const kLazyAreaId = "test-886323-lazy-area-for-removability-testing";
+
+var gNavBar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+var gLazyArea;
+
+// Removable nodes shouldn't be moved by buildArea
+add_task(async function () {
+  let dummyBtn = createDummyXULButton(kButtonId, "Dummy");
+  dummyBtn.setAttribute("removable", "true");
+  CustomizableUI.getCustomizationTarget(gNavBar).appendChild(dummyBtn);
+  let popupSet = document.getElementById("mainPopupSet");
+  gLazyArea = document.createXULElement("panel");
+  gLazyArea.id = kLazyAreaId;
+  gLazyArea.hidden = true;
+  popupSet.appendChild(gLazyArea);
+  CustomizableUI.registerArea(kLazyAreaId, {
+    type: CustomizableUI.TYPE_PANEL,
+    defaultPlacements: [],
+  });
+  CustomizableUI.addWidgetToArea(kButtonId, kLazyAreaId);
+  assertAreaPlacements(
+    kLazyAreaId,
+    [kButtonId],
+    "Placements should have changed because widget is removable."
+  );
+  let btn = document.getElementById(kButtonId);
+  btn.setAttribute("removable", "false");
+  gLazyArea._customizationTarget = gLazyArea;
+  CustomizableUI.registerToolbarNode(gLazyArea, []);
+  assertAreaPlacements(
+    kLazyAreaId,
+    [],
+    "Placements should no longer include widget."
+  );
+  is(
+    btn.parentNode.id,
+    CustomizableUI.getCustomizationTarget(gNavBar).id,
+    "Button shouldn't actually have moved as it's not removable"
+  );
+  btn = document.getElementById(kButtonId);
+  if (btn) {
+    btn.remove();
+  }
+  CustomizableUI.removeWidgetFromArea(kButtonId);
+  CustomizableUI.unregisterArea(kLazyAreaId);
+  gLazyArea.remove();
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_890262_destroyWidget_after_add_to_panel.js b/browser/components/customizableui/test/browser_890262_destroyWidget_after_add_to_panel.js
new file mode 100644
index 0000000000..61e354dd61
--- /dev/null
+++ b/browser/components/customizableui/test/browser_890262_destroyWidget_after_add_to_panel.js
@@ -0,0 +1,74 @@
+/* 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";
+
+const kLazyAreaId = "test-890262-lazy-area";
+const kWidget1Id = "test-890262-widget1";
+const kWidget2Id = "test-890262-widget2";
+
+setupArea();
+
+// Destroying a widget after defaulting it to a lazy area should work.
+add_task(function () {
+  CustomizableUI.createWidget({
+    id: kWidget1Id,
+    removable: true,
+    defaultArea: kLazyAreaId,
+  });
+  let noError = true;
+  try {
+    CustomizableUI.destroyWidget(kWidget1Id);
+  } catch (ex) {
+    console.error(ex);
+    noError = false;
+  }
+  ok(
+    noError,
+    "Shouldn't throw an exception for a widget that was created in a not-yet-constructed area"
+  );
+});
+
+// Destroying a widget after moving it to a lazy area should work.
+add_task(function () {
+  CustomizableUI.createWidget({
+    id: kWidget2Id,
+    removable: true,
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  });
+
+  CustomizableUI.addWidgetToArea(kWidget2Id, kLazyAreaId);
+  let noError = true;
+  try {
+    CustomizableUI.destroyWidget(kWidget2Id);
+  } catch (ex) {
+    console.error(ex);
+    noError = false;
+  }
+  ok(
+    noError,
+    "Shouldn't throw an exception for a widget that was added to a not-yet-constructed area"
+  );
+});
+
+add_task(async function asyncCleanup() {
+  let lazyArea = document.getElementById(kLazyAreaId);
+  if (lazyArea) {
+    lazyArea.remove();
+  }
+  try {
+    CustomizableUI.unregisterArea(kLazyAreaId);
+  } catch (ex) {} // If we didn't register successfully for some reason
+  await resetCustomization();
+});
+
+function setupArea() {
+  let lazyArea = document.createXULElement("hbox");
+  lazyArea.id = kLazyAreaId;
+  document.getElementById("nav-bar").appendChild(lazyArea);
+  CustomizableUI.registerArea(kLazyAreaId, {
+    type: CustomizableUI.TYPE_TOOLBAR,
+    defaultPlacements: [],
+  });
+}
diff --git a/browser/components/customizableui/test/browser_892955_isWidgetRemovable_for_removed_widgets.js b/browser/components/customizableui/test/browser_892955_isWidgetRemovable_for_removed_widgets.js
new file mode 100644
index 0000000000..7584c52bb6
--- /dev/null
+++ b/browser/components/customizableui/test/browser_892955_isWidgetRemovable_for_removed_widgets.js
@@ -0,0 +1,30 @@
+/* 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";
+
+const kWidgetId = "test-892955-remove-widget";
+
+// Removing a destroyed widget should work.
+add_task(async function () {
+  let widgetSpec = {
+    id: kWidgetId,
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  };
+
+  CustomizableUI.createWidget(widgetSpec);
+  CustomizableUI.destroyWidget(kWidgetId);
+  let noError = true;
+  try {
+    CustomizableUI.removeWidgetFromArea(kWidgetId);
+  } catch (ex) {
+    noError = false;
+    console.error(ex);
+  }
+  ok(noError, "Shouldn't throw an error removing a destroyed widget.");
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_892956_destroyWidget_defaultPlacements.js b/browser/components/customizableui/test/browser_892956_destroyWidget_defaultPlacements.js
new file mode 100644
index 0000000000..7ad68e16d0
--- /dev/null
+++ b/browser/components/customizableui/test/browser_892956_destroyWidget_defaultPlacements.js
@@ -0,0 +1,30 @@
+/* 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";
+
+const kWidgetId = "test-892956-destroyWidget-defaultPlacement";
+
+// destroyWidget should clean up defaultPlacements if the widget had a defaultArea
+add_task(async function () {
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be in the default state when we start"
+  );
+
+  let widgetSpec = {
+    id: kWidgetId,
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  };
+  CustomizableUI.createWidget(widgetSpec);
+  CustomizableUI.destroyWidget(kWidgetId);
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be in the default state when we finish"
+  );
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_901207_searchbar_in_panel.js b/browser/components/customizableui/test/browser_901207_searchbar_in_panel.js
new file mode 100644
index 0000000000..1576e10cec
--- /dev/null
+++ b/browser/components/customizableui/test/browser_901207_searchbar_in_panel.js
@@ -0,0 +1,139 @@
+/* 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";
+
+logActiveElement();
+
+async function waitForSearchBarFocus() {
+  let searchbar = document.getElementById("searchbar");
+  await TestUtils.waitForCondition(function () {
+    logActiveElement();
+    return document.activeElement === searchbar.textbox;
+  });
+}
+
+// Ctrl+K should open the menu panel and focus the search bar if the search bar is in the panel.
+add_task(async function check_shortcut_when_in_closed_overflow_panel_closed() {
+  CustomizableUI.addWidgetToArea(
+    "search-container",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  let shownPanelPromise = promiseOverflowShown(window);
+  sendWebSearchKeyCommand();
+  await shownPanelPromise;
+
+  await waitForSearchBarFocus();
+
+  let hiddenPanelPromise = promiseOverflowHidden(window);
+  EventUtils.synthesizeKey("KEY_Escape");
+  await hiddenPanelPromise;
+  CustomizableUI.reset();
+});
+
+// Ctrl+K should give focus to the searchbar when the searchbar is in the menupanel and the panel is already opened.
+add_task(async function check_shortcut_when_in_opened_overflow_panel() {
+  CustomizableUI.addWidgetToArea(
+    "search-container",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await document.getElementById("nav-bar").overflowable.show();
+
+  sendWebSearchKeyCommand();
+
+  await waitForSearchBarFocus();
+
+  let hiddenPanelPromise = promiseOverflowHidden(window);
+  EventUtils.synthesizeKey("KEY_Escape");
+  await hiddenPanelPromise;
+  CustomizableUI.reset();
+});
+
+// Ctrl+K should open the overflow panel and focus the search bar if the search bar is overflowed.
+add_task(async function check_shortcut_when_in_overflow() {
+  this.originalWindowWidth = window.outerWidth;
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start with a non-overflowing toolbar."
+  );
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+
+  Services.prefs.setBoolPref("browser.search.widget.inNavBar", true);
+
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() => {
+    return (
+      navbar.getAttribute("overflowing") == "true" &&
+      !navbar.querySelector("#search-container")
+    );
+  });
+  ok(
+    !navbar.querySelector("#search-container"),
+    "Search container should be overflowing"
+  );
+
+  let shownPanelPromise = promiseOverflowShown(window);
+  sendWebSearchKeyCommand();
+  await shownPanelPromise;
+
+  let chevron = document.getElementById("nav-bar-overflow-button");
+  await TestUtils.waitForCondition(() => chevron.open);
+
+  await waitForSearchBarFocus();
+
+  let hiddenPanelPromise = promiseOverflowHidden(window);
+  EventUtils.synthesizeKey("KEY_Escape");
+  await hiddenPanelPromise;
+
+  Services.prefs.setBoolPref("browser.search.widget.inNavBar", false);
+
+  navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  window.resizeTo(this.originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should not have an overflowing toolbar."
+  );
+});
+
+// Ctrl+K should focus the search bar if it is in the navbar and not overflowing.
+add_task(async function check_shortcut_when_not_in_overflow() {
+  Services.prefs.setBoolPref("browser.search.widget.inNavBar", true);
+  let placement = CustomizableUI.getPlacementOfWidget("search-container");
+  is(placement.area, CustomizableUI.AREA_NAVBAR, "Should be in nav-bar");
+
+  sendWebSearchKeyCommand();
+
+  // This fails if the screen resolution is small and the search bar overflows
+  // from the nav bar even with the original window width.
+  await waitForSearchBarFocus();
+
+  Services.prefs.setBoolPref("browser.search.widget.inNavBar", false);
+});
+
+function sendWebSearchKeyCommand() {
+  document.documentElement.focus();
+  EventUtils.synthesizeKey("k", { accelKey: true });
+}
+
+function logActiveElement() {
+  let element = document.activeElement;
+  let str = "";
+  while (element && element.parentNode) {
+    str =
+      " (" +
+      element.localName +
+      "#" +
+      element.id +
+      "." +
+      [...element.classList].join(".") +
+      ") >" +
+      str;
+    element = element.parentNode;
+  }
+  info("Active element: " + element ? str : "null");
+}
diff --git a/browser/components/customizableui/test/browser_909779_overflow_toolbars_new_window.js b/browser/components/customizableui/test/browser_909779_overflow_toolbars_new_window.js
new file mode 100644
index 0000000000..bca9a05b94
--- /dev/null
+++ b/browser/components/customizableui/test/browser_909779_overflow_toolbars_new_window.js
@@ -0,0 +1,49 @@
+/* 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";
+
+// Resize to a small window, open a new window, check that new window handles overflow properly
+add_task(async function () {
+  let originalWindowWidth = window.outerWidth;
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start with a non-overflowing toolbar."
+  );
+  let oldChildCount =
+    CustomizableUI.getCustomizationTarget(navbar).childElementCount;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() => navbar.hasAttribute("overflowing"));
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+
+  ok(
+    CustomizableUI.getCustomizationTarget(navbar).childElementCount <
+      oldChildCount,
+    "Should have fewer children."
+  );
+  let newWindow = await openAndLoadWindow();
+  let otherNavBar = newWindow.document.getElementById(
+    CustomizableUI.AREA_NAVBAR
+  );
+  await TestUtils.waitForCondition(() =>
+    otherNavBar.hasAttribute("overflowing")
+  );
+  ok(
+    otherNavBar.hasAttribute("overflowing"),
+    "Other window should have an overflowing toolbar."
+  );
+  await promiseWindowClosed(newWindow);
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should no longer have an overflowing toolbar."
+  );
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_913972_currentset_overflow.js b/browser/components/customizableui/test/browser_913972_currentset_overflow.js
new file mode 100644
index 0000000000..fde37e4f3c
--- /dev/null
+++ b/browser/components/customizableui/test/browser_913972_currentset_overflow.js
@@ -0,0 +1,91 @@
+/* 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";
+
+var navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+registerCleanupFunction(async function asyncCleanup() {
+  await resetCustomization();
+});
+
+// Resize to a small window, resize back, shouldn't affect default state.
+add_task(async function () {
+  let originalWindowWidth = window.outerWidth;
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start with a non-overflowing toolbar."
+  );
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+  let navbarTarget = CustomizableUI.getCustomizationTarget(navbar);
+  let oldChildCount = navbarTarget.childElementCount;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () => navbar.hasAttribute("overflowing"),
+    "Navbar has a overflowing attribute"
+  );
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should still be in default state when overflowing."
+  );
+  ok(
+    navbarTarget.childElementCount < oldChildCount,
+    "Should have fewer children."
+  );
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () => !navbar.hasAttribute("overflowing"),
+    "Navbar does not have an overflowing attribute"
+  );
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should no longer have an overflowing toolbar."
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should still be in default state now we're no longer overflowing."
+  );
+
+  // Verify actual physical placements match those of the placement array:
+  let placementCounter = 0;
+  let placements = CustomizableUI.getWidgetIdsInArea(
+    CustomizableUI.AREA_NAVBAR
+  );
+  for (let node of navbarTarget.children) {
+    if (node.getAttribute("skipintoolbarset") == "true") {
+      continue;
+    }
+    is(
+      placements[placementCounter++],
+      node.id,
+      "Nodes should match after overflow"
+    );
+  }
+  is(
+    placements.length,
+    placementCounter,
+    "Should have as many nodes as expected"
+  );
+  is(
+    navbarTarget.childElementCount,
+    oldChildCount,
+    "Number of nodes should match"
+  );
+});
+
+// Enter and exit customization mode, check that default state is correct.
+add_task(async function () {
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+  await startCustomizing();
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be in default state in customization mode."
+  );
+  await endCustomizing();
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be in default state after customization mode."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_914138_widget_API_overflowable_toolbar.js b/browser/components/customizableui/test/browser_914138_widget_API_overflowable_toolbar.js
new file mode 100644
index 0000000000..1d57e3d1fb
--- /dev/null
+++ b/browser/components/customizableui/test/browser_914138_widget_API_overflowable_toolbar.js
@@ -0,0 +1,347 @@
+/* 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";
+
+var navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+var overflowList = document.getElementById(
+  navbar.getAttribute("default-overflowtarget")
+);
+
+const kTestBtn1 = "test-addWidgetToArea-overflow";
+const kTestBtn2 = "test-removeWidgetFromArea-overflow";
+const kTestBtn3 = "test-createWidget-overflow";
+const kTestBtn4 = "test-createWidget-overflow-first-item";
+const kTestBtn5 = "test-addWidgetToArea-overflow-first-item";
+const kSidebarBtn = "sidebar-button";
+const kLibraryButton = "library-button";
+const kDownloadsBtn = "downloads-button";
+const kSearchBox = "search-container";
+
+var originalWindowWidth;
+
+// Adding a widget should add it next to the widget it's being inserted next to.
+add_task(async function subsequent_widget() {
+  originalWindowWidth = window.outerWidth;
+  createDummyXULButton(kTestBtn1, "Test");
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start subsequent_widget with a non-overflowing toolbar."
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should start subsequent_widget in default state."
+  );
+  CustomizableUI.addWidgetToArea(kSidebarBtn, "nav-bar");
+  await waitForElementShown(document.getElementById(kSidebarBtn));
+
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() => {
+    return (
+      navbar.hasAttribute("overflowing") &&
+      document.getElementById(kSidebarBtn).getAttribute("overflowedItem") ==
+        "true"
+    );
+  });
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+  ok(
+    !navbar.querySelector("#" + kSidebarBtn),
+    "Sidebar button should no longer be in the navbar"
+  );
+  let sidebarBtnNode = overflowList.querySelector("#" + kSidebarBtn);
+  ok(sidebarBtnNode, "Sidebar button should be overflowing");
+  ok(
+    sidebarBtnNode && sidebarBtnNode.getAttribute("overflowedItem") == "true",
+    "Sidebar button should have overflowedItem attribute"
+  );
+
+  let placementOfSidebarButton = CustomizableUI.getWidgetIdsInArea(
+    navbar.id
+  ).indexOf(kSidebarBtn);
+  CustomizableUI.addWidgetToArea(
+    kTestBtn1,
+    navbar.id,
+    placementOfSidebarButton
+  );
+  ok(
+    !navbar.querySelector("#" + kTestBtn1),
+    "New button should not be in the navbar"
+  );
+  let newButtonNode = overflowList.querySelector("#" + kTestBtn1);
+  ok(newButtonNode, "New button should be overflowing");
+  ok(
+    newButtonNode && newButtonNode.getAttribute("overflowedItem") == "true",
+    "New button should have overflowedItem attribute"
+  );
+  let nextEl = newButtonNode && newButtonNode.nextElementSibling;
+  is(
+    nextEl && nextEl.id,
+    kSidebarBtn,
+    "Test button should be next to sidebar button."
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should not have an overflowing toolbar."
+  );
+  ok(
+    navbar.querySelector("#" + kSidebarBtn),
+    "Sidebar button should be in the navbar"
+  );
+  ok(
+    sidebarBtnNode && sidebarBtnNode.getAttribute("overflowedItem") != "true",
+    "Sidebar button should no longer have overflowedItem attribute"
+  );
+  ok(
+    !overflowList.querySelector("#" + kSidebarBtn),
+    "Sidebar button should no longer be overflowing"
+  );
+  ok(
+    navbar.querySelector("#" + kTestBtn1),
+    "Test button should be in the navbar"
+  );
+  ok(
+    !overflowList.querySelector("#" + kTestBtn1),
+    "Test button should no longer be overflowing"
+  );
+  ok(
+    newButtonNode && newButtonNode.getAttribute("overflowedItem") != "true",
+    "New button should no longer have overflowedItem attribute"
+  );
+  let el = document.getElementById(kTestBtn1);
+  if (el) {
+    CustomizableUI.removeWidgetFromArea(kTestBtn1);
+    el.remove();
+  }
+  CustomizableUI.removeWidgetFromArea(kSidebarBtn);
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+// Removing a widget should remove it from the overflow list if that is where it is, and update it accordingly.
+add_task(async function remove_widget() {
+  createDummyXULButton(kTestBtn2, "Test");
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start remove_widget with a non-overflowing toolbar."
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should start remove_widget in default state."
+  );
+  CustomizableUI.addWidgetToArea(kTestBtn2, navbar.id);
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should still have a non-overflowing toolbar."
+  );
+
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() => navbar.hasAttribute("overflowing"));
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+  ok(
+    !navbar.querySelector("#" + kTestBtn2),
+    "Test button should not be in the navbar"
+  );
+  ok(
+    overflowList.querySelector("#" + kTestBtn2),
+    "Test button should be overflowing"
+  );
+
+  CustomizableUI.removeWidgetFromArea(kTestBtn2);
+
+  ok(
+    !overflowList.querySelector("#" + kTestBtn2),
+    "Test button should not be overflowing."
+  );
+  ok(
+    !navbar.querySelector("#" + kTestBtn2),
+    "Test button should not be in the navbar"
+  );
+  ok(
+    gNavToolbox.palette.querySelector("#" + kTestBtn2),
+    "Test button should be in the palette"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should not have an overflowing toolbar."
+  );
+  let el = document.getElementById(kTestBtn2);
+  if (el) {
+    CustomizableUI.removeWidgetFromArea(kTestBtn2);
+    el.remove();
+  }
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+// Constructing a widget while overflown should set the right class on it.
+add_task(async function construct_widget() {
+  originalWindowWidth = window.outerWidth;
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start construct_widget with a non-overflowing toolbar."
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should start construct_widget in default state."
+  );
+
+  CustomizableUI.addWidgetToArea(kSidebarBtn, "nav-bar");
+  await waitForElementShown(document.getElementById(kSidebarBtn));
+
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() => {
+    return (
+      navbar.hasAttribute("overflowing") &&
+      document.getElementById(kSidebarBtn).getAttribute("overflowedItem") ==
+        "true"
+    );
+  });
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+  ok(
+    !navbar.querySelector("#" + kSidebarBtn),
+    "Sidebar button should no longer be in the navbar"
+  );
+  let sidebarBtnNode = overflowList.querySelector("#" + kSidebarBtn);
+  ok(sidebarBtnNode, "Sidebar button should be overflowing");
+  ok(
+    sidebarBtnNode && sidebarBtnNode.getAttribute("overflowedItem") == "true",
+    "Sidebar button should have overflowedItem class"
+  );
+
+  let testBtnSpec = {
+    id: kTestBtn3,
+    label: "Overflowable widget test",
+    defaultArea: "nav-bar",
+  };
+  CustomizableUI.createWidget(testBtnSpec);
+  let testNode = overflowList.querySelector("#" + kTestBtn3);
+  ok(testNode, "Test button should be overflowing");
+  ok(
+    testNode && testNode.getAttribute("overflowedItem") == "true",
+    "Test button should have overflowedItem class"
+  );
+
+  CustomizableUI.destroyWidget(kTestBtn3);
+  testNode = document.getElementById(kTestBtn3);
+  ok(!testNode, "Test button should be gone");
+
+  CustomizableUI.createWidget(testBtnSpec);
+  testNode = overflowList.querySelector("#" + kTestBtn3);
+  ok(testNode, "Test button should be overflowing");
+  ok(
+    testNode && testNode.getAttribute("overflowedItem") == "true",
+    "Test button should have overflowedItem class"
+  );
+
+  CustomizableUI.removeWidgetFromArea(kTestBtn3);
+  testNode = document.getElementById(kTestBtn3);
+  ok(!testNode, "Test button should be gone");
+  CustomizableUI.destroyWidget(kTestBtn3);
+  CustomizableUI.removeWidgetFromArea(kSidebarBtn);
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+add_task(async function insertBeforeFirstItemInOverflow() {
+  originalWindowWidth = window.outerWidth;
+
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start insertBeforeFirstItemInOverflow with a non-overflowing toolbar."
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should start insertBeforeFirstItemInOverflow in default state."
+  );
+
+  CustomizableUI.addWidgetToArea(
+    kLibraryButton,
+    "nav-bar",
+    CustomizableUI.getWidgetIdsInArea("nav-bar").indexOf(
+      "save-to-pocket-button"
+    )
+  );
+  let libraryButton = document.getElementById(kLibraryButton);
+  await waitForElementShown(libraryButton);
+  // Ensure nothing flexes to make the resize predictable:
+  navbar
+    .querySelectorAll("toolbarspring")
+    .forEach(s => CustomizableUI.removeWidgetFromArea(s.id));
+  let urlbar = document.getElementById("urlbar-container");
+  urlbar.style.minWidth = urlbar.getBoundingClientRect().width + "px";
+  // Negative number to make the window smaller by the difference between the left side of
+  // the item next to the library button and left side of the hamburger one.
+  // The width of the overflow button that needs to appear will then be enough to
+  // also hide the library button.
+  let resizeWidthToMakeLibraryLast =
+    libraryButton.nextElementSibling.getBoundingClientRect().left -
+    PanelUI.menuButton.parentNode.getBoundingClientRect().left +
+    10; // Leave some margin for the margins between buttons etc.;
+  info(
+    "Resizing to " +
+      resizeWidthToMakeLibraryLast +
+      " , waiting for library to overflow."
+  );
+  window.resizeBy(resizeWidthToMakeLibraryLast, 0);
+  await TestUtils.waitForCondition(() => {
+    return (
+      libraryButton.getAttribute("overflowedItem") == "true" &&
+      !libraryButton.previousElementSibling
+    );
+  });
+
+  let testBtnSpec = { id: kTestBtn4, label: "Overflowable widget test" };
+  let placementOfLibraryButton = CustomizableUI.getWidgetIdsInArea(
+    navbar.id
+  ).indexOf(kLibraryButton);
+  CustomizableUI.createWidget(testBtnSpec);
+  CustomizableUI.addWidgetToArea(
+    kTestBtn4,
+    "nav-bar",
+    placementOfLibraryButton
+  );
+  let testNode = overflowList.querySelector("#" + kTestBtn4);
+  ok(testNode, "Test button should be overflowing");
+  ok(
+    testNode && testNode.getAttribute("overflowedItem") == "true",
+    "Test button should have overflowedItem class"
+  );
+  CustomizableUI.destroyWidget(kTestBtn4);
+  testNode = document.getElementById(kTestBtn4);
+  ok(!testNode, "Test button should be gone");
+
+  createDummyXULButton(kTestBtn5, "Test");
+  CustomizableUI.addWidgetToArea(
+    kTestBtn5,
+    "nav-bar",
+    placementOfLibraryButton
+  );
+  testNode = overflowList.querySelector("#" + kTestBtn5);
+  ok(testNode, "Test button should be overflowing");
+  ok(
+    testNode && testNode.getAttribute("overflowedItem") == "true",
+    "Test button should have overflowedItem class"
+  );
+  CustomizableUI.removeWidgetFromArea(kTestBtn5);
+  testNode && testNode.remove();
+
+  urlbar.style.removeProperty("min-width");
+  CustomizableUI.removeWidgetFromArea(kLibraryButton);
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  await resetCustomization();
+});
+
+registerCleanupFunction(async function asyncCleanup() {
+  document.getElementById("urlbar-container").style.removeProperty("min-width");
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_918049_skipintoolbarset_dnd.js b/browser/components/customizableui/test/browser_918049_skipintoolbarset_dnd.js
new file mode 100644
index 0000000000..bd0a7d5795
--- /dev/null
+++ b/browser/components/customizableui/test/browser_918049_skipintoolbarset_dnd.js
@@ -0,0 +1,46 @@
+/* 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";
+
+var navbar;
+var skippedItem;
+
+// Attempting to drag a skipintoolbarset item should work.
+add_task(async function () {
+  navbar = document.getElementById("nav-bar");
+  skippedItem = document.createXULElement("toolbarbutton");
+  skippedItem.id = "test-skipintoolbarset-item";
+  skippedItem.setAttribute("label", "Test");
+  skippedItem.setAttribute("skipintoolbarset", "true");
+  skippedItem.setAttribute("removable", "true");
+  CustomizableUI.getCustomizationTarget(navbar).appendChild(skippedItem);
+  let stopReloadButton = document.getElementById("stop-reload-button");
+  await startCustomizing();
+  await waitForElementShown(skippedItem);
+  ok(CustomizableUI.inDefaultState, "Should still be in default state");
+  simulateItemDrag(skippedItem, stopReloadButton, "start", 0);
+  ok(CustomizableUI.inDefaultState, "Should still be in default state");
+  let skippedItemWrapper = skippedItem.parentNode;
+  is(
+    skippedItemWrapper.nextElementSibling &&
+      skippedItemWrapper.nextElementSibling.id,
+    stopReloadButton.parentNode.id,
+    "Should be next to stop/reload button"
+  );
+  simulateItemDrag(stopReloadButton, skippedItem, "start", 0);
+  let wrapper = stopReloadButton.parentNode;
+  is(
+    wrapper.nextElementSibling && wrapper.nextElementSibling.id,
+    skippedItem.parentNode.id,
+    "Should be next to skipintoolbarset item"
+  );
+  ok(CustomizableUI.inDefaultState, "Should still be in default state");
+});
+
+add_task(async function asyncCleanup() {
+  await endCustomizing();
+  skippedItem.remove();
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_923857_customize_mode_event_wrapping_during_reset.js b/browser/components/customizableui/test/browser_923857_customize_mode_event_wrapping_during_reset.js
new file mode 100644
index 0000000000..d416f34144
--- /dev/null
+++ b/browser/components/customizableui/test/browser_923857_customize_mode_event_wrapping_during_reset.js
@@ -0,0 +1,27 @@
+/* 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";
+
+// Customize mode reset button should revert correctly
+add_task(async function () {
+  await startCustomizing();
+  let devButton = document.getElementById("developer-button");
+  let fxaButton = document.getElementById("fxa-toolbar-menu-button");
+  let stopReloadButton = document.getElementById("stop-reload-button");
+  let palette = document.getElementById("customization-palette");
+  ok(
+    devButton && fxaButton && stopReloadButton && palette,
+    "Stuff should exist"
+  );
+  simulateItemDrag(devButton, fxaButton);
+  simulateItemDrag(stopReloadButton, palette);
+  await gCustomizeMode.reset();
+  ok(CustomizableUI.inDefaultState, "Should be back in default state");
+  await endCustomizing();
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_927717_customize_drag_empty_toolbar.js b/browser/components/customizableui/test/browser_927717_customize_drag_empty_toolbar.js
new file mode 100644
index 0000000000..340e840d83
--- /dev/null
+++ b/browser/components/customizableui/test/browser_927717_customize_drag_empty_toolbar.js
@@ -0,0 +1,29 @@
+/* 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";
+
+const kTestToolbarId = "test-empty-drag";
+
+// Attempting to drag an item to an empty container should work.
+add_task(async function () {
+  await createToolbarWithPlacements(kTestToolbarId, []);
+  await startCustomizing();
+  let libraryButton = document.getElementById("library-button");
+  let customToolbar = document.getElementById(kTestToolbarId);
+  simulateItemDrag(libraryButton, customToolbar);
+  assertAreaPlacements(kTestToolbarId, ["library-button"]);
+  ok(
+    libraryButton.parentNode &&
+      libraryButton.parentNode.parentNode == customToolbar,
+    "Button should really be in toolbar"
+  );
+  await endCustomizing();
+  removeCustomToolbars();
+});
+
+add_task(async function asyncCleanup() {
+  await endCustomizing();
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_934113_menubar_removable.js b/browser/components/customizableui/test/browser_934113_menubar_removable.js
new file mode 100644
index 0000000000..8f41baba7a
--- /dev/null
+++ b/browser/components/customizableui/test/browser_934113_menubar_removable.js
@@ -0,0 +1,43 @@
+/* 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";
+
+// Attempting to drag the menubar to the navbar shouldn't work.
+add_task(async function () {
+  await startCustomizing();
+  let menuItems = document.getElementById("menubar-items");
+  let navbar = document.getElementById("nav-bar");
+  let menubar = document.getElementById("toolbar-menubar");
+  // Force the menu to be shown.
+  const kAutohide = menubar.getAttribute("autohide");
+  menubar.setAttribute("autohide", "false");
+  simulateItemDrag(menuItems, CustomizableUI.getCustomizationTarget(navbar));
+
+  is(
+    getAreaWidgetIds("nav-bar").indexOf("menubar-items"),
+    -1,
+    "Menu bar shouldn't be in the navbar."
+  );
+  ok(
+    !navbar.querySelector("#menubar-items"),
+    "Shouldn't find menubar items in the navbar."
+  );
+  ok(
+    menubar.querySelector("#menubar-items"),
+    "Should find menubar items in the menubar."
+  );
+  isnot(
+    getAreaWidgetIds("toolbar-menubar").indexOf("menubar-items"),
+    -1,
+    "Menubar items shouldn't be missing from the navbar."
+  );
+  menubar.setAttribute("autohide", kAutohide);
+  await endCustomizing();
+});
+
+add_task(async function asyncCleanup() {
+  await endCustomizing();
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_934951_zoom_in_toolbar.js b/browser/components/customizableui/test/browser_934951_zoom_in_toolbar.js
new file mode 100644
index 0000000000..db1d6175ab
--- /dev/null
+++ b/browser/components/customizableui/test/browser_934951_zoom_in_toolbar.js
@@ -0,0 +1,94 @@
+/* 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/. */
+/* eslint-disable mozilla/no-arbitrary-setTimeout */
+
+"use strict";
+
+let gZoomResetButton;
+
+async function waitForZoom(zoom) {
+  if (parseInt(gZoomResetButton.label) == zoom) {
+    return;
+  }
+  await promiseAttributeMutation(gZoomResetButton, "label", v => {
+    return parseInt(v) == zoom;
+  });
+}
+
+// Bug 934951 - Zoom controls percentage label doesn't update when it's in the toolbar and you navigate.
+add_task(async function () {
+  CustomizableUI.addWidgetToArea("zoom-controls", CustomizableUI.AREA_NAVBAR);
+  gZoomResetButton = document.getElementById("zoom-reset-button");
+  let tab1 = BrowserTestUtils.addTab(gBrowser, "about:mozilla");
+  await BrowserTestUtils.browserLoaded(tab1.linkedBrowser);
+  let tab2 = BrowserTestUtils.addTab(gBrowser, "about:robots");
+  await BrowserTestUtils.browserLoaded(tab2.linkedBrowser);
+  gBrowser.selectedTab = tab1;
+
+  registerCleanupFunction(() => {
+    info("Cleaning up.");
+    CustomizableUI.reset();
+    gBrowser.removeTab(tab2);
+    gBrowser.removeTab(tab1);
+  });
+
+  is(
+    parseInt(gZoomResetButton.label, 10),
+    100,
+    "Default zoom is 100% for about:mozilla"
+  );
+  FullZoom.enlarge();
+  await waitForZoom(110);
+  is(
+    parseInt(gZoomResetButton.label, 10),
+    110,
+    "Zoom is changed to 110% for about:mozilla"
+  );
+
+  let tabSelectPromise = TestUtils.topicObserved(
+    "browser-fullZoom:location-change"
+  );
+  gBrowser.selectedTab = tab2;
+  await tabSelectPromise;
+  await waitForZoom(100);
+  is(
+    parseInt(gZoomResetButton.label, 10),
+    100,
+    "Default zoom is 100% for about:robots"
+  );
+
+  gBrowser.selectedTab = tab1;
+  await waitForZoom(110);
+  FullZoom.reset();
+  await waitForZoom(100);
+  is(
+    parseInt(gZoomResetButton.label, 10),
+    100,
+    "Default zoom is 100% for about:mozilla"
+  );
+
+  // Test zoom label updates while navigating pages in the same tab.
+  FullZoom.enlarge();
+  await waitForZoom(110);
+  is(
+    parseInt(gZoomResetButton.label, 10),
+    110,
+    "Zoom is changed to 110% for about:mozilla"
+  );
+  await promiseTabLoadEvent(tab1, "about:home");
+  await waitForZoom(100);
+  is(
+    parseInt(gZoomResetButton.label, 10),
+    100,
+    "Default zoom is 100% for about:home"
+  );
+  gBrowser.selectedBrowser.goBack();
+  await waitForZoom(110);
+  is(
+    parseInt(gZoomResetButton.label, 10),
+    110,
+    "Zoom is still 110% for about:mozilla"
+  );
+  FullZoom.reset();
+});
diff --git a/browser/components/customizableui/test/browser_938980_navbar_collapsed.js b/browser/components/customizableui/test/browser_938980_navbar_collapsed.js
new file mode 100644
index 0000000000..28b75c0a37
--- /dev/null
+++ b/browser/components/customizableui/test/browser_938980_navbar_collapsed.js
@@ -0,0 +1,214 @@
+/* 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";
+
+requestLongerTimeout(2);
+
+var bookmarksToolbar = document.getElementById("PersonalToolbar");
+var navbar = document.getElementById("nav-bar");
+var tabsToolbar = document.getElementById("TabsToolbar");
+
+// Customization reset should restore visibility to default-visible toolbars.
+add_task(async function () {
+  is(navbar.collapsed, false, "Test should start with navbar visible");
+  setToolbarVisibility(navbar, false);
+  is(navbar.collapsed, true, "navbar should be hidden now");
+
+  await resetCustomization();
+
+  is(
+    navbar.collapsed,
+    false,
+    "Customization reset should restore visibility to the navbar"
+  );
+});
+
+// Customization reset should restore collapsed-state to default-collapsed toolbars.
+add_task(async function () {
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state"
+  );
+
+  is(
+    bookmarksToolbar.collapsed,
+    true,
+    "Test should start with bookmarks toolbar collapsed"
+  );
+  ok(bookmarksToolbar.collapsed, "bookmarksToolbar should be collapsed");
+  ok(!tabsToolbar.collapsed, "TabsToolbar should not be collapsed");
+  is(navbar.collapsed, false, "The nav-bar should be shown by default");
+
+  setToolbarVisibility(bookmarksToolbar, true);
+  setToolbarVisibility(navbar, false);
+  ok(!bookmarksToolbar.collapsed, "bookmarksToolbar should be visible now");
+  ok(navbar.collapsed, "navbar should be collapsed");
+  is(
+    CustomizableUI.inDefaultState,
+    false,
+    "Should no longer be in default state"
+  );
+
+  await startCustomizing();
+  await gCustomizeMode.reset();
+  await endCustomizing();
+
+  is(
+    bookmarksToolbar.collapsed,
+    true,
+    "Customization reset should restore collapsed-state to the bookmarks toolbar"
+  );
+  ok(!tabsToolbar.collapsed, "TabsToolbar should not be collapsed");
+  ok(
+    bookmarksToolbar.collapsed,
+    "The bookmarksToolbar should be collapsed after reset"
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be back to default state"
+  );
+});
+
+// Check that the menubar will be collapsed by resetting, if the platform supports it.
+add_task(async function () {
+  let menubar = document.getElementById("toolbar-menubar");
+  const canMenubarCollapse = CustomizableUI.isToolbarDefaultCollapsed(
+    menubar.id
+  );
+  if (!canMenubarCollapse) {
+    return;
+  }
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state"
+  );
+
+  is(
+    menubar.getBoundingClientRect().height,
+    0,
+    "menubar should be hidden by default"
+  );
+  setToolbarVisibility(menubar, true);
+  isnot(
+    menubar.getBoundingClientRect().height,
+    0,
+    "menubar should be visible now"
+  );
+
+  await startCustomizing();
+  await gCustomizeMode.reset();
+
+  is(
+    menubar.getAttribute("autohide"),
+    "true",
+    "The menubar should have autohide=true after reset in customization mode"
+  );
+  is(
+    menubar.getBoundingClientRect().height,
+    0,
+    "The menubar should have height=0 after reset in customization mode"
+  );
+
+  await endCustomizing();
+
+  is(
+    menubar.getAttribute("autohide"),
+    "true",
+    "The menubar should have autohide=true after reset"
+  );
+  is(
+    menubar.getBoundingClientRect().height,
+    0,
+    "The menubar should have height=0 after reset"
+  );
+});
+
+// Customization reset should restore collapsed-state to default-collapsed toolbars.
+add_task(async function () {
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state"
+  );
+  ok(bookmarksToolbar.collapsed, "bookmarksToolbar should be collapsed");
+  ok(!tabsToolbar.collapsed, "TabsToolbar should not be collapsed");
+
+  setToolbarVisibility(bookmarksToolbar, true);
+  ok(!bookmarksToolbar.collapsed, "bookmarksToolbar should be visible now");
+  is(
+    CustomizableUI.inDefaultState,
+    false,
+    "Should no longer be in default state"
+  );
+
+  await startCustomizing();
+
+  ok(
+    !bookmarksToolbar.collapsed,
+    "The bookmarksToolbar should be visible before reset"
+  );
+  ok(!navbar.collapsed, "The navbar should be visible before reset");
+  ok(!tabsToolbar.collapsed, "TabsToolbar should not be collapsed");
+
+  await gCustomizeMode.reset();
+
+  ok(
+    bookmarksToolbar.collapsed,
+    "The bookmarksToolbar should be collapsed after reset"
+  );
+  ok(!tabsToolbar.collapsed, "TabsToolbar should not be collapsed");
+  ok(!navbar.collapsed, "The navbar should still be visible after reset");
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be back to default state"
+  );
+  await endCustomizing();
+});
+
+// Check that the menubar will be collapsed by resetting, if the platform supports it.
+add_task(async function () {
+  let menubar = document.getElementById("toolbar-menubar");
+  const canMenubarCollapse = CustomizableUI.isToolbarDefaultCollapsed(
+    menubar.id
+  );
+  if (!canMenubarCollapse) {
+    return;
+  }
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state"
+  );
+  await startCustomizing();
+  let resetButton = document.getElementById("customization-reset-button");
+  is(
+    resetButton.disabled,
+    true,
+    "The reset button should be disabled when in default state"
+  );
+
+  setToolbarVisibility(menubar, true);
+  is(
+    resetButton.disabled,
+    false,
+    "The reset button should be enabled when not in default state"
+  );
+  ok(
+    !CustomizableUI.inDefaultState,
+    "No longer in default state when the menubar is shown"
+  );
+
+  await gCustomizeMode.reset();
+
+  is(
+    resetButton.disabled,
+    true,
+    "The reset button should be disabled when in default state"
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state"
+  );
+
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_938995_indefaultstate_nonremovable.js b/browser/components/customizableui/test/browser_938995_indefaultstate_nonremovable.js
new file mode 100644
index 0000000000..2d926e1725
--- /dev/null
+++ b/browser/components/customizableui/test/browser_938995_indefaultstate_nonremovable.js
@@ -0,0 +1,45 @@
+/* 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";
+
+const kWidgetId = "test-non-removable-widget";
+
+// Adding non-removable items to a toolbar or the panel shouldn't change inDefaultState
+add_task(async function () {
+  ok(CustomizableUI.inDefaultState, "Should start in default state");
+
+  let button = createDummyXULButton(
+    kWidgetId,
+    "Test non-removable inDefaultState handling"
+  );
+  CustomizableUI.addWidgetToArea(kWidgetId, CustomizableUI.AREA_NAVBAR);
+  button.setAttribute("removable", "false");
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should still be in default state after navbar addition"
+  );
+  button.remove();
+
+  button = createDummyXULButton(
+    kWidgetId,
+    "Test non-removable inDefaultState handling"
+  );
+  CustomizableUI.addWidgetToArea(
+    kWidgetId,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  button.setAttribute("removable", "false");
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should still be in default state after panel addition"
+  );
+  button.remove();
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be in default state after destroying both widgets"
+  );
+  // reset now that button is gone.
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_940013_registerToolbarNode_calls_registerArea.js b/browser/components/customizableui/test/browser_940013_registerToolbarNode_calls_registerArea.js
new file mode 100644
index 0000000000..c4fa54f782
--- /dev/null
+++ b/browser/components/customizableui/test/browser_940013_registerToolbarNode_calls_registerArea.js
@@ -0,0 +1,64 @@
+/* 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";
+
+const kToolbarId = "test-registerToolbarNode-toolbar";
+const kButtonId = "test-registerToolbarNode-button";
+registerCleanupFunction(cleanup);
+
+// Registering a toolbar without a defaultset attribute should
+// wait for the registerArea call
+add_task(async function () {
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state."
+  );
+  let btn = createDummyXULButton(kButtonId);
+  let toolbar = document.createXULElement("toolbar");
+  toolbar.id = kToolbarId;
+  toolbar.setAttribute("customizable", true);
+  gNavToolbox.appendChild(toolbar);
+  CustomizableUI.registerToolbarNode(toolbar);
+  ok(
+    !CustomizableUI.areas.includes(kToolbarId),
+    "Toolbar should not yet have been registered automatically."
+  );
+  CustomizableUI.registerArea(kToolbarId, { defaultPlacements: [kButtonId] });
+  ok(
+    CustomizableUI.areas.includes(kToolbarId),
+    "Toolbar should have been registered now."
+  );
+  is(
+    CustomizableUI.getAreaType(kToolbarId),
+    CustomizableUI.TYPE_TOOLBAR,
+    "Area should be registered as toolbar"
+  );
+  assertAreaPlacements(kToolbarId, [kButtonId]);
+  ok(
+    !CustomizableUI.inDefaultState,
+    "No longer in default state after toolbar is registered and visible."
+  );
+  CustomizableUI.unregisterArea(kToolbarId, true);
+  toolbar.remove();
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state."
+  );
+  btn.remove();
+});
+
+async function cleanup() {
+  let toolbar = document.getElementById(kToolbarId);
+  if (toolbar) {
+    toolbar.remove();
+  }
+  let btn =
+    document.getElementById(kButtonId) ||
+    gNavToolbox.querySelector("#" + kButtonId);
+  if (btn) {
+    btn.remove();
+  }
+  await resetCustomization();
+}
diff --git a/browser/components/customizableui/test/browser_940307_panel_click_closure_handling.js b/browser/components/customizableui/test/browser_940307_panel_click_closure_handling.js
new file mode 100644
index 0000000000..38a50d6c24
--- /dev/null
+++ b/browser/components/customizableui/test/browser_940307_panel_click_closure_handling.js
@@ -0,0 +1,134 @@
+/* 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";
+
+var button, menuButton;
+/* Clicking a button should close the panel */
+add_task(async function plain_button() {
+  button = document.createXULElement("toolbarbutton");
+  button.id = "browser_940307_button";
+  button.setAttribute("label", "Button");
+  gNavToolbox.palette.appendChild(button);
+  CustomizableUI.addWidgetToArea(
+    button.id,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  await document.getElementById("nav-bar").overflowable.show();
+  let hiddenAgain = promiseOverflowHidden(window);
+  EventUtils.synthesizeMouseAtCenter(button, {});
+  await hiddenAgain;
+  CustomizableUI.removeWidgetFromArea(button.id);
+  button.remove();
+});
+
+add_task(async function searchbar_in_panel() {
+  CustomizableUI.addWidgetToArea(
+    "search-container",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  await document.getElementById("nav-bar").overflowable.show();
+
+  let searchbar = document.getElementById("searchbar");
+  await TestUtils.waitForCondition(
+    () => "value" in searchbar && searchbar.value === ""
+  );
+
+  // Focusing a non-empty searchbox will cause us to open the
+  // autocomplete panel and search for suggestions, which would
+  // trigger network requests. Temporarily disable suggestions.
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.search.suggest.enabled", false]],
+  });
+  let dontShowPopup = e => e.preventDefault();
+  let searchbarPopup = searchbar.textbox.popup;
+  searchbarPopup.addEventListener("popupshowing", dontShowPopup);
+
+  searchbar.value = "foo";
+  searchbar.focus();
+
+  // Can't use promisePanelElementShown() here since the search bar
+  // creates its context menu lazily the first time it is opened.
+  let contextMenuShown = new Promise(resolve => {
+    let listener = event => {
+      if (searchbar._menupopup && event.target == searchbar._menupopup) {
+        window.removeEventListener("popupshown", listener);
+        resolve(searchbar._menupopup);
+      }
+    };
+    window.addEventListener("popupshown", listener);
+  });
+  EventUtils.synthesizeMouseAtCenter(searchbar, {
+    type: "contextmenu",
+    button: 2,
+  });
+  let contextmenu = await contextMenuShown;
+
+  ok(isOverflowOpen(), "Panel should still be open");
+
+  let selectAll = contextmenu.querySelector("[cmd='cmd_selectAll']");
+  let contextMenuHidden = promisePanelElementHidden(window, contextmenu);
+  contextmenu.activateItem(selectAll);
+  await contextMenuHidden;
+
+  ok(isOverflowOpen(), "Panel should still be open");
+
+  let hiddenPanelPromise = promiseOverflowHidden(window);
+  EventUtils.synthesizeKey("KEY_Escape");
+  await hiddenPanelPromise;
+  ok(!isOverflowOpen(), "Panel should no longer be open");
+
+  // Allow search bar popup to show again.
+  searchbarPopup.removeEventListener("popupshowing", dontShowPopup);
+
+  // We focused the search bar earlier - ensure we don't keep doing that.
+  gURLBar.select();
+
+  CustomizableUI.reset();
+});
+
+add_task(async function disabled_button_in_panel() {
+  button = document.createXULElement("toolbarbutton");
+  button.id = "browser_946166_button_disabled";
+  button.setAttribute("disabled", "true");
+  button.setAttribute("label", "Button");
+  gNavToolbox.palette.appendChild(button);
+  CustomizableUI.addWidgetToArea(
+    button.id,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  await document.getElementById("nav-bar").overflowable.show();
+  EventUtils.synthesizeMouseAtCenter(button, {});
+  is(PanelUI.overflowPanel.state, "open", "Popup stays open");
+  button.removeAttribute("disabled");
+  let hiddenAgain = promiseOverflowHidden(window);
+  EventUtils.synthesizeMouseAtCenter(button, {});
+  await hiddenAgain;
+  button.remove();
+});
+
+registerCleanupFunction(function () {
+  if (button && button.parentNode) {
+    button.remove();
+  }
+  if (menuButton && menuButton.parentNode) {
+    menuButton.remove();
+  }
+  // Sadly this isn't task.jsm-enabled, so we can't wait for this to happen. But we should
+  // definitely close it here and hope it won't interfere with other tests.
+  // Of course, all the tests are meant to do this themselves, but if they fail...
+  if (isOverflowOpen()) {
+    PanelUI.overflowPanel.hidePopup();
+  }
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_940946_removable_from_navbar_customizemode.js b/browser/components/customizableui/test/browser_940946_removable_from_navbar_customizemode.js
new file mode 100644
index 0000000000..2144dd2483
--- /dev/null
+++ b/browser/components/customizableui/test/browser_940946_removable_from_navbar_customizemode.js
@@ -0,0 +1,30 @@
+/* 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";
+
+const kTestBtnId = "test-removable-navbar-customize-mode";
+
+// Items without the removable attribute in the navbar should be considered non-removable
+add_task(async function () {
+  let btn = createDummyXULButton(
+    kTestBtnId,
+    "Test removable in navbar in customize mode"
+  );
+  CustomizableUI.getCustomizationTarget(
+    document.getElementById("nav-bar")
+  ).appendChild(btn);
+  await startCustomizing();
+  ok(
+    !CustomizableUI.isWidgetRemovable(kTestBtnId),
+    "Widget should not be considered removable"
+  );
+  await endCustomizing();
+  document.getElementById(kTestBtnId).remove();
+});
+
+add_task(async function asyncCleanup() {
+  await endCustomizing();
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_941083_invalidate_wrapper_cache_createWidget.js b/browser/components/customizableui/test/browser_941083_invalidate_wrapper_cache_createWidget.js
new file mode 100644
index 0000000000..6df5084849
--- /dev/null
+++ b/browser/components/customizableui/test/browser_941083_invalidate_wrapper_cache_createWidget.js
@@ -0,0 +1,49 @@
+/* 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";
+
+// See https://bugzilla.mozilla.org/show_bug.cgi?id=941083
+
+const kWidgetId = "test-invalidate-wrapper-cache";
+
+// Check createWidget invalidates the widget cache
+add_task(function () {
+  let groupWrapper = CustomizableUI.getWidget(kWidgetId);
+  ok(groupWrapper, "Should get group wrapper.");
+  let singleWrapper = groupWrapper.forWindow(window);
+  ok(singleWrapper, "Should get single wrapper.");
+
+  CustomizableUI.createWidget({
+    id: kWidgetId,
+    label: "Test invalidating widgets caching",
+  });
+
+  let newGroupWrapper = CustomizableUI.getWidget(kWidgetId);
+  ok(newGroupWrapper, "Should get a group wrapper again.");
+  isnot(newGroupWrapper, groupWrapper, "Wrappers shouldn't be the same.");
+  isnot(
+    newGroupWrapper.provider,
+    groupWrapper.provider,
+    "Wrapper providers shouldn't be the same."
+  );
+
+  let newSingleWrapper = newGroupWrapper.forWindow(window);
+  isnot(
+    newSingleWrapper,
+    singleWrapper,
+    "Single wrappers shouldn't be the same."
+  );
+  isnot(
+    newSingleWrapper.provider,
+    singleWrapper.provider,
+    "Single wrapper providers shouldn't be the same."
+  );
+
+  CustomizableUI.destroyWidget(kWidgetId);
+  ok(
+    !CustomizableUI.getWidget(kWidgetId),
+    "Shouldn't get a wrapper after destroying the widget."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_942581_unregisterArea_keeps_placements.js b/browser/components/customizableui/test/browser_942581_unregisterArea_keeps_placements.js
new file mode 100644
index 0000000000..6f18d590b7
--- /dev/null
+++ b/browser/components/customizableui/test/browser_942581_unregisterArea_keeps_placements.js
@@ -0,0 +1,119 @@
+/* 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";
+
+const kToolbarName = "test-unregisterArea-placements-toolbar";
+const kTestWidgetPfx = "test-widget-for-unregisterArea-placements-";
+const kTestWidgetCount = 3;
+registerCleanupFunction(removeCustomToolbars);
+
+// unregisterArea should keep placements by default and restore them when re-adding the area
+add_task(async function () {
+  let widgetIds = [];
+  for (let i = 0; i < kTestWidgetCount; i++) {
+    let id = kTestWidgetPfx + i;
+    widgetIds.push(id);
+    let spec = {
+      id,
+      type: "button",
+      removable: true,
+      label: "unregisterArea test",
+      tooltiptext: "" + i,
+    };
+    CustomizableUI.createWidget(spec);
+  }
+  for (let i = kTestWidgetCount; i < kTestWidgetCount * 2; i++) {
+    let id = kTestWidgetPfx + i;
+    widgetIds.push(id);
+    createDummyXULButton(id, "unregisterArea XUL test " + i);
+  }
+  let toolbarNode = createToolbarWithPlacements(kToolbarName, widgetIds);
+  checkAbstractAndRealPlacements(toolbarNode, widgetIds);
+
+  // Now move one of them:
+  CustomizableUI.moveWidgetWithinArea(kTestWidgetPfx + kTestWidgetCount, 0);
+  // Clone the array so we know this is the modified one:
+  let modifiedWidgetIds = [...widgetIds];
+  let movedWidget = modifiedWidgetIds.splice(kTestWidgetCount, 1)[0];
+  modifiedWidgetIds.unshift(movedWidget);
+
+  // Check it:
+  checkAbstractAndRealPlacements(toolbarNode, modifiedWidgetIds);
+
+  // Then unregister
+  CustomizableUI.unregisterArea(kToolbarName);
+
+  // Check we tell the outside world no dangerous things:
+  checkWidgetFates(widgetIds);
+  // Only then remove the real node
+  toolbarNode.remove();
+
+  // Now move one of the items to the palette, and another to the navbar:
+  let lastWidget = modifiedWidgetIds.pop();
+  CustomizableUI.removeWidgetFromArea(lastWidget);
+  lastWidget = modifiedWidgetIds.pop();
+  CustomizableUI.addWidgetToArea(lastWidget, CustomizableUI.AREA_NAVBAR);
+
+  // Recreate ourselves with the default placements being the same:
+  toolbarNode = createToolbarWithPlacements(kToolbarName, widgetIds);
+  // Then check that after doing this, our actual placements match
+  // the modified list, not the default one.
+  checkAbstractAndRealPlacements(toolbarNode, modifiedWidgetIds);
+
+  // Now remove completely:
+  CustomizableUI.unregisterArea(kToolbarName, true);
+  checkWidgetFates(modifiedWidgetIds);
+  toolbarNode.remove();
+
+  // One more time:
+  // Recreate ourselves with the default placements being the same:
+  toolbarNode = createToolbarWithPlacements(kToolbarName, widgetIds);
+  // Should now be back to default:
+  checkAbstractAndRealPlacements(toolbarNode, widgetIds);
+  CustomizableUI.unregisterArea(kToolbarName, true);
+  checkWidgetFates(widgetIds);
+  toolbarNode.remove();
+
+  // XXXgijs: ensure cleanup function doesn't barf:
+  gAddedToolbars.delete(kToolbarName);
+
+  // Remove all the XUL widgets, destroy the others:
+  for (let widget of widgetIds) {
+    let widgetWrapper = CustomizableUI.getWidget(widget);
+    if (widgetWrapper.provider == CustomizableUI.PROVIDER_XUL) {
+      gNavToolbox.palette.querySelector("#" + widget).remove();
+    } else {
+      CustomizableUI.destroyWidget(widget);
+    }
+  }
+});
+
+function checkAbstractAndRealPlacements(aNode, aExpectedPlacements) {
+  assertAreaPlacements(kToolbarName, aExpectedPlacements);
+  let physicalWidgetIds = Array.from(aNode.children, node => node.id);
+  placementArraysEqual(aNode.id, physicalWidgetIds, aExpectedPlacements);
+}
+
+function checkWidgetFates(aWidgetIds) {
+  for (let widget of aWidgetIds) {
+    ok(
+      !CustomizableUI.getPlacementOfWidget(widget),
+      "Widget should be in palette"
+    );
+    ok(!document.getElementById(widget), "Widget should not be in the DOM");
+    let widgetInPalette = !!gNavToolbox.palette.querySelector("#" + widget);
+    let widgetProvider = CustomizableUI.getWidget(widget).provider;
+    let widgetIsXULWidget = widgetProvider == CustomizableUI.PROVIDER_XUL;
+    is(
+      widgetInPalette,
+      widgetIsXULWidget,
+      "Just XUL Widgets should be in the palette"
+    );
+  }
+}
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_944887_destroyWidget_should_destroy_in_palette.js b/browser/components/customizableui/test/browser_944887_destroyWidget_should_destroy_in_palette.js
new file mode 100644
index 0000000000..3da14ab217
--- /dev/null
+++ b/browser/components/customizableui/test/browser_944887_destroyWidget_should_destroy_in_palette.js
@@ -0,0 +1,26 @@
+/* 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";
+
+const kWidgetId = "test-destroy-in-palette";
+
+// Check destroyWidget destroys the node if it's in the palette
+add_task(async function () {
+  CustomizableUI.createWidget({
+    id: kWidgetId,
+    label: "Test destroying widgets in palette.",
+  });
+  await startCustomizing();
+  await endCustomizing();
+  ok(
+    gNavToolbox.palette.querySelector("#" + kWidgetId),
+    "Widget still exists in palette."
+  );
+  CustomizableUI.destroyWidget(kWidgetId);
+  ok(
+    !gNavToolbox.palette.querySelector("#" + kWidgetId),
+    "Widget no longer exists in palette."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_945739_showInPrivateBrowsing_customize_mode.js b/browser/components/customizableui/test/browser_945739_showInPrivateBrowsing_customize_mode.js
new file mode 100644
index 0000000000..bac97aed3b
--- /dev/null
+++ b/browser/components/customizableui/test/browser_945739_showInPrivateBrowsing_customize_mode.js
@@ -0,0 +1,42 @@
+/* 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";
+
+const kWidgetId = "test-private-browsing-customize-mode-widget";
+
+// Add a widget via the API with showInPrivateBrowsing set to false
+// and ensure it does not appear in the list of unused widgets in private
+// windows.
+add_task(async function testPrivateBrowsingCustomizeModeWidget() {
+  CustomizableUI.createWidget({
+    id: kWidgetId,
+    showInPrivateBrowsing: false,
+  });
+
+  let normalWidgetArray = CustomizableUI.getUnusedWidgets(gNavToolbox.palette);
+  normalWidgetArray = normalWidgetArray.map(w => w.id);
+  ok(
+    normalWidgetArray.indexOf(kWidgetId) > -1,
+    "Widget should appear as unused in non-private window"
+  );
+
+  let privateWindow = await openAndLoadWindow({ private: true });
+  let privateWidgetArray = CustomizableUI.getUnusedWidgets(
+    privateWindow.gNavToolbox.palette
+  );
+  privateWidgetArray = privateWidgetArray.map(w => w.id);
+  is(
+    privateWidgetArray.indexOf(kWidgetId),
+    -1,
+    "Widget should not appear as unused in private window"
+  );
+  await promiseWindowClosed(privateWindow);
+
+  CustomizableUI.destroyWidget(kWidgetId);
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_copy.js b/browser/components/customizableui/test/browser_947914_button_copy.js
new file mode 100644
index 0000000000..e6e0e287c4
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_copy.js
@@ -0,0 +1,64 @@
+/* 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";
+
+var initialLocation = gBrowser.currentURI.spec;
+var globalClipboard;
+
+add_task(async function () {
+  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
+      );
+
+      await waitForOverflowButtonShown();
+
+      let testText = "copy text test";
+
+      gURLBar.focus();
+      info("The URL bar was focused");
+      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.valueIsTyped = true;
+      gURLBar.focus();
+      gURLBar.select();
+      await document.getElementById("nav-bar").overflowable.show();
+      info("Menu panel was opened");
+
+      ok(
+        !copyButton.hasAttribute("disabled"),
+        "Copy button is enabled when selecting"
+      );
+
+      await SimpleTest.promiseClipboardChange(testText, () => {
+        copyButton.click();
+      });
+
+      is(
+        gURLBar.value,
+        testText,
+        "Selected text is unaltered when clicking copy"
+      );
+    }
+  );
+});
+
+registerCleanupFunction(function cleanup() {
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_cut.js b/browser/components/customizableui/test/browser_947914_button_cut.js
new file mode 100644
index 0000000000..3ea5622b51
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_cut.js
@@ -0,0 +1,58 @@
+/* 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";
+
+var initialLocation = gBrowser.currentURI.spec;
+var globalClipboard;
+
+add_task(async function () {
+  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
+      );
+
+      await waitForOverflowButtonShown();
+
+      let testText = "cut text test";
+
+      gURLBar.focus();
+      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.valueIsTyped = true;
+      gURLBar.focus();
+      gURLBar.select();
+      await document.getElementById("nav-bar").overflowable.show();
+      info("Menu panel was opened");
+
+      ok(
+        !cutButton.hasAttribute("disabled"),
+        "Cut button is enabled when selecting"
+      );
+      await SimpleTest.promiseClipboardChange(testText, () => {
+        cutButton.click();
+      });
+      is(
+        gURLBar.value,
+        "",
+        "Selected text is removed from source when clicking on cut"
+      );
+    }
+  );
+});
+
+registerCleanupFunction(function cleanup() {
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_find.js b/browser/components/customizableui/test/browser_947914_button_find.js
new file mode 100644
index 0000000000..c767239d9d
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_find.js
@@ -0,0 +1,37 @@
+/* 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 () {
+  info("Check find button existence and functionality");
+  // The TabContextMenu initializes its strings only on a focus or mouseover event.
+  // Calls focus event on the TabContextMenu early in the test.
+  gBrowser.selectedTab.focus();
+  CustomizableUI.addWidgetToArea(
+    "find-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await waitForOverflowButtonShown();
+
+  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");
+
+  let findBarPromise = gBrowser.isFindBarInitialized()
+    ? null
+    : BrowserTestUtils.waitForEvent(gBrowser.selectedTab, "TabFindInitialized");
+
+  findButton.click();
+  await findBarPromise;
+  ok(!gFindBar.hasAttribute("hidden"), "Findbar opened successfully");
+
+  // close find bar
+  gFindBar.close();
+  info("Findbar was closed");
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_history.js b/browser/components/customizableui/test/browser_947914_button_history.js
new file mode 100644
index 0000000000..d4ad28c04f
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_history.js
@@ -0,0 +1,68 @@
+/* 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";
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "http://example.com"
+);
+
+add_task(async function () {
+  info("Check history button existence and functionality");
+  // The TabContextMenu initializes its strings only on a focus or mouseover event.
+  // Calls focus event on the TabContextMenu early in the test.
+  gBrowser.selectedTab.focus();
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_PATH + "dummy_history_item.html"
+  );
+  BrowserTestUtils.removeTab(tab);
+
+  tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PATH); // will 404, but we don't care.
+
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await waitForOverflowButtonShown();
+
+  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");
+
+  historyButton.click();
+
+  let historyPanel = document.getElementById("PanelUI-history");
+  let promise = BrowserTestUtils.waitForEvent(historyPanel, "ViewShown");
+  await promise;
+  ok(historyPanel.getAttribute("visible"), "History Panel is in view");
+
+  let browserLoaded = BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  let panelHiddenPromise = promiseOverflowHidden(window);
+
+  let historyItems = document.getElementById("appMenu_historyMenu");
+  let historyItemForURL = historyItems.querySelector(
+    "toolbarbutton.bookmark-item[label='Happy History Hero']"
+  );
+  ok(
+    historyItemForURL,
+    "Should have a history item for the history we just made."
+  );
+  EventUtils.synthesizeMouseAtCenter(historyItemForURL, {});
+  await browserLoaded;
+  is(
+    gBrowser.currentURI.spec,
+    TEST_PATH + "dummy_history_item.html",
+    "Should have expected page load"
+  );
+
+  await panelHiddenPromise;
+  BrowserTestUtils.removeTab(tab);
+  info("Menu panel was closed");
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_newPrivateWindow.js b/browser/components/customizableui/test/browser_947914_button_newPrivateWindow.js
new file mode 100644
index 0000000000..cc8842a3e8
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_newPrivateWindow.js
@@ -0,0 +1,62 @@
+/* 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 () {
+  info("Check private browsing button existence and functionality");
+  CustomizableUI.addWidgetToArea(
+    "privatebrowsing-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await waitForOverflowButtonShown();
+
+  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") {
+        privateWindow = aSubject;
+        privateWindow.addEventListener(
+          "load",
+          function () {
+            is(
+              privateWindow.location.href,
+              AppConstants.BROWSER_CHROME_URL,
+              "A new browser window was opened"
+            );
+            ok(
+              PrivateBrowsingUtils.isWindowPrivate(privateWindow),
+              "Window is private"
+            );
+            windowWasHandled = true;
+          },
+          { once: true }
+        );
+      }
+    },
+  };
+
+  Services.ww.registerNotification(observerWindowOpened);
+
+  let privateBrowsingButton = document.getElementById("privatebrowsing-button");
+  ok(privateBrowsingButton, "Private browsing button exists in Panel Menu");
+  privateBrowsingButton.click();
+
+  try {
+    await TestUtils.waitForCondition(() => windowWasHandled);
+    await promiseWindowClosed(privateWindow);
+    info("The new private window was closed");
+  } catch (e) {
+    ok(false, "The new private browser window was not properly handled");
+  } finally {
+    Services.ww.unregisterNotification(observerWindowOpened);
+  }
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_newWindow.js b/browser/components/customizableui/test/browser_947914_button_newWindow.js
new file mode 100644
index 0000000000..591d13191e
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_newWindow.js
@@ -0,0 +1,62 @@
+/* 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 () {
+  info("Check new window button existence and functionality");
+  CustomizableUI.addWidgetToArea(
+    "new-window-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  await waitForOverflowButtonShown();
+
+  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") {
+        newWindow = aSubject;
+        newWindow.addEventListener(
+          "load",
+          function () {
+            is(
+              newWindow.location.href,
+              AppConstants.BROWSER_CHROME_URL,
+              "A new browser window was opened"
+            );
+            ok(
+              !PrivateBrowsingUtils.isWindowPrivate(newWindow),
+              "Window is not private"
+            );
+            windowWasHandled = true;
+          },
+          { once: true }
+        );
+      }
+    },
+  };
+
+  Services.ww.registerNotification(observerWindowOpened);
+
+  let newWindowButton = document.getElementById("new-window-button");
+  ok(newWindowButton, "New Window button exists in Panel Menu");
+  newWindowButton.click();
+
+  try {
+    await TestUtils.waitForCondition(() => windowWasHandled);
+    await promiseWindowClosed(newWindow);
+    info("The new window was closed");
+  } catch (e) {
+    ok(false, "The new browser window was not properly handled");
+  } finally {
+    Services.ww.unregisterNotification(observerWindowOpened);
+  }
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_paste.js b/browser/components/customizableui/test/browser_947914_button_paste.js
new file mode 100644
index 0000000000..a5d107faa6
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_paste.js
@@ -0,0 +1,55 @@
+/* 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";
+
+var initialLocation = gBrowser.currentURI.spec;
+var globalClipboard;
+
+add_task(async function () {
+  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 waitForOverflowButtonShown();
+
+      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);
+
+      // test paste button by pasting text to URL bar
+      gURLBar.focus();
+      await gCUITestUtils.openMainMenu();
+      info("Menu panel was opened");
+
+      ok(!pasteButton.hasAttribute("disabled"), "Paste button is enabled");
+      pasteButton.click();
+
+      is(gURLBar.value, text, "Text pasted successfully");
+
+      await gCUITestUtils.hideMainMenu();
+    }
+  );
+});
+
+registerCleanupFunction(function cleanup() {
+  CustomizableUI.reset();
+  Services.clipboard.emptyClipboard(globalClipboard);
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_print.js b/browser/components/customizableui/test/browser_947914_button_print.js
new file mode 100644
index 0000000000..96d2328b2d
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_print.js
@@ -0,0 +1,54 @@
+/* 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";
+
+const isOSX = Services.appinfo.OS === "Darwin";
+
+add_task(async function () {
+  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 waitForOverflowButtonShown();
+
+      await document.getElementById("nav-bar").overflowable.show();
+      info("Menu panel was opened");
+
+      await TestUtils.waitForCondition(
+        () => document.getElementById("print-button") != null
+      );
+
+      let printButton = document.getElementById("print-button");
+      ok(printButton, "Print button exists in Panel Menu");
+
+      printButton.click();
+
+      // Ensure we're showing the preview...
+      await BrowserTestUtils.waitForCondition(() => {
+        let preview = document.querySelector(".printPreviewBrowser");
+        return preview && BrowserTestUtils.is_visible(preview);
+      });
+
+      ok(true, "Entered print preview mode");
+
+      gBrowser.getTabDialogBox(gBrowser.selectedBrowser).abortAllDialogs();
+      // Wait for the preview to go away
+      await BrowserTestUtils.waitForCondition(
+        () => !document.querySelector(".printPreviewBrowser")
+      );
+
+      info("Exited print preview");
+    }
+  );
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_zoomIn.js b/browser/components/customizableui/test/browser_947914_button_zoomIn.js
new file mode 100644
index 0000000000..361a1f53fb
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_zoomIn.js
@@ -0,0 +1,60 @@
+/* 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 () {
+  info("Check zoom in button existence and functionality");
+
+  is(ZoomManager.zoom, 1, "Initial zoom factor should be 1");
+
+  CustomizableUI.addWidgetToArea(
+    "zoom-controls",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  registerCleanupFunction(async () => {
+    CustomizableUI.reset();
+    let gContentPrefs = Cc["@mozilla.org/content-pref/service;1"].getService(
+      Ci.nsIContentPrefService2
+    );
+    let gLoadContext = Cu.createLoadContext();
+    await new Promise(resolve => {
+      gContentPrefs.removeByName(window.FullZoom.name, gLoadContext, {
+        handleResult() {},
+        handleCompletion() {
+          resolve();
+        },
+      });
+    });
+  });
+
+  await waitForOverflowButtonShown();
+
+  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);
+  info("Page zoom level is: " + pageZoomLevel);
+
+  let zoomResetButton = document.getElementById("zoom-reset-button");
+  await TestUtils.waitForCondition(() => {
+    info(
+      "Current zoom is " + parseInt(zoomResetButton.getAttribute("label"), 10)
+    );
+    return parseInt(zoomResetButton.getAttribute("label"), 10) == pageZoomLevel;
+  });
+
+  ok(pageZoomLevel > 100, "Page zoomed in correctly");
+
+  // close the Panel
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  document.getElementById("widget-overflow").hidePopup();
+  await panelHiddenPromise;
+  info("Menu panel was closed");
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_zoomOut.js b/browser/components/customizableui/test/browser_947914_button_zoomOut.js
new file mode 100644
index 0000000000..b6175bccf5
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_zoomOut.js
@@ -0,0 +1,61 @@
+/* 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 () {
+  info("Check zoom out button existence and functionality");
+
+  is(ZoomManager.zoom, 1, "Initial zoom factor should be 1");
+
+  CustomizableUI.addWidgetToArea(
+    "zoom-controls",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  registerCleanupFunction(async () => {
+    CustomizableUI.reset();
+    let gContentPrefs = Cc["@mozilla.org/content-pref/service;1"].getService(
+      Ci.nsIContentPrefService2
+    );
+    let gLoadContext = Cu.createLoadContext();
+    await new Promise(resolve => {
+      gContentPrefs.removeByName(window.FullZoom.name, gLoadContext, {
+        handleResult() {},
+        handleCompletion() {
+          resolve();
+        },
+      });
+    });
+  });
+
+  await waitForOverflowButtonShown();
+
+  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);
+  console.log("Page zoom level is: ", pageZoomLevel);
+
+  let zoomResetButton = document.getElementById("zoom-reset-button");
+  await TestUtils.waitForCondition(() => {
+    console.log(
+      "Current zoom is ",
+      parseInt(zoomResetButton.getAttribute("label"), 10)
+    );
+    return parseInt(zoomResetButton.getAttribute("label"), 10) == pageZoomLevel;
+  });
+
+  ok(pageZoomLevel < 100, "Page zoomed out correctly");
+
+  // close the panel
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  document.getElementById("widget-overflow").hidePopup();
+  await panelHiddenPromise;
+  info("Menu panel was closed");
+});
diff --git a/browser/components/customizableui/test/browser_947914_button_zoomReset.js b/browser/components/customizableui/test/browser_947914_button_zoomReset.js
new file mode 100644
index 0000000000..7dc8299b28
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947914_button_zoomReset.js
@@ -0,0 +1,75 @@
+/* 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";
+
+var initialPageZoom = ZoomManager.zoom;
+
+add_task(async function () {
+  info("Check zoom reset button existence and functionality");
+  is(initialPageZoom, 1, "Page zoom reset correctly");
+
+  await BrowserTestUtils.withNewTab(
+    { gBrowser, url: "http://example.com", waitForLoad: true },
+    async function (browser) {
+      CustomizableUI.addWidgetToArea(
+        "zoom-controls",
+        CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+      );
+
+      registerCleanupFunction(() => CustomizableUI.reset());
+
+      CustomizableUI.addWidgetToArea(
+        "zoom-controls",
+        CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+      );
+
+      await waitForOverflowButtonShown();
+
+      {
+        let zoomChange = BrowserTestUtils.waitForEvent(
+          gBrowser,
+          "FullZoomChange"
+        );
+        ZoomManager.zoom = 0.5;
+        await zoomChange;
+      }
+
+      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");
+
+      let zoomChange = BrowserTestUtils.waitForEvent(
+        gBrowser,
+        "FullZoomChange"
+      );
+      zoomResetButton.click();
+      await zoomChange;
+
+      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 = 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");
+});
diff --git a/browser/components/customizableui/test/browser_947987_removable_default.js b/browser/components/customizableui/test/browser_947987_removable_default.js
new file mode 100644
index 0000000000..84bbd3ed59
--- /dev/null
+++ b/browser/components/customizableui/test/browser_947987_removable_default.js
@@ -0,0 +1,94 @@
+/* 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";
+
+var kWidgetId = "test-removable-widget-default";
+const kNavBar = CustomizableUI.AREA_NAVBAR;
+var widgetCounter = 0;
+
+registerCleanupFunction(removeCustomToolbars);
+
+// Sanity checks
+add_task(function () {
+  let brokenSpec = { id: kWidgetId + widgetCounter++, removable: false };
+  SimpleTest.doesThrow(
+    () => CustomizableUI.createWidget(brokenSpec),
+    "Creating non-removable widget without defaultArea should throw."
+  );
+
+  // Widget without removable set should be removable:
+  let wrapper = CustomizableUI.createWidget({
+    id: kWidgetId + widgetCounter++,
+  });
+  ok(
+    CustomizableUI.isWidgetRemovable(wrapper.id),
+    "Should be removable by default."
+  );
+  CustomizableUI.destroyWidget(wrapper.id);
+});
+
+// Test non-removable widget with defaultArea
+add_task(async function () {
+  // Non-removable widget with defaultArea should work:
+  let spec = {
+    id: kWidgetId + widgetCounter++,
+    removable: false,
+    defaultArea: kNavBar,
+  };
+  let widgetWrapper;
+  try {
+    widgetWrapper = CustomizableUI.createWidget(spec);
+  } catch (ex) {
+    ok(
+      false,
+      "Creating a non-removable widget with a default area should not throw."
+    );
+    return;
+  }
+
+  let placement = CustomizableUI.getPlacementOfWidget(spec.id);
+  ok(placement, "Widget should be placed.");
+  is(placement.area, kNavBar, "Widget should be in navbar");
+  let singleWrapper = widgetWrapper.forWindow(window);
+  ok(singleWrapper, "Widget should exist in window.");
+  ok(singleWrapper.node, "Widget node should exist in window.");
+  let expectedParent = CustomizableUI.getCustomizeTargetForArea(
+    kNavBar,
+    window
+  );
+  is(
+    singleWrapper.node.parentNode,
+    expectedParent,
+    "Widget should be in navbar."
+  );
+
+  let otherWin = await openAndLoadWindow(true);
+  placement = CustomizableUI.getPlacementOfWidget(spec.id);
+  ok(placement, "Widget should be placed.");
+  is(placement && placement.area, kNavBar, "Widget should be in navbar");
+
+  singleWrapper = widgetWrapper.forWindow(otherWin);
+  ok(singleWrapper, "Widget should exist in other window.");
+  if (singleWrapper) {
+    ok(singleWrapper.node, "Widget node should exist in other window.");
+    if (singleWrapper.node) {
+      let expectedParentInOtherWin = CustomizableUI.getCustomizeTargetForArea(
+        kNavBar,
+        otherWin
+      );
+      is(
+        singleWrapper.node.parentNode,
+        expectedParentInOtherWin,
+        "Widget should be in navbar in other window."
+      );
+    }
+  }
+  CustomizableUI.destroyWidget(spec.id);
+  await promiseWindowClosed(otherWin);
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_948985_non_removable_defaultArea.js b/browser/components/customizableui/test/browser_948985_non_removable_defaultArea.js
new file mode 100644
index 0000000000..ce8d3c2d3a
--- /dev/null
+++ b/browser/components/customizableui/test/browser_948985_non_removable_defaultArea.js
@@ -0,0 +1,47 @@
+/* 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/. */
+
+const kWidgetId = "test-destroy-non-removable-defaultArea";
+
+add_task(function () {
+  let spec = {
+    id: kWidgetId,
+    label: "Test non-removable defaultArea re-adding.",
+    removable: false,
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  };
+  CustomizableUI.createWidget(spec);
+  let placement = CustomizableUI.getPlacementOfWidget(kWidgetId);
+  ok(placement, "Should have placed the widget.");
+  is(
+    placement && placement.area,
+    CustomizableUI.AREA_NAVBAR,
+    "Widget should be in navbar"
+  );
+  CustomizableUI.destroyWidget(kWidgetId);
+  CustomizableUI.removeWidgetFromArea(kWidgetId);
+
+  CustomizableUI.createWidget(spec);
+  ok(placement, "Should have placed the widget.");
+  is(
+    placement && placement.area,
+    CustomizableUI.AREA_NAVBAR,
+    "Widget should be in navbar"
+  );
+  CustomizableUI.destroyWidget(kWidgetId);
+  CustomizableUI.removeWidgetFromArea(kWidgetId);
+
+  const kPrefCustomizationAutoAdd = "browser.uiCustomization.autoAdd";
+  Services.prefs.setBoolPref(kPrefCustomizationAutoAdd, false);
+  CustomizableUI.createWidget(spec);
+  ok(placement, "Should have placed the widget.");
+  is(
+    placement && placement.area,
+    CustomizableUI.AREA_NAVBAR,
+    "Widget should be in navbar"
+  );
+  CustomizableUI.destroyWidget(kWidgetId);
+  CustomizableUI.removeWidgetFromArea(kWidgetId);
+  Services.prefs.clearUserPref(kPrefCustomizationAutoAdd);
+});
diff --git a/browser/components/customizableui/test/browser_952963_areaType_getter_no_area.js b/browser/components/customizableui/test/browser_952963_areaType_getter_no_area.js
new file mode 100644
index 0000000000..1dab702fc8
--- /dev/null
+++ b/browser/components/customizableui/test/browser_952963_areaType_getter_no_area.js
@@ -0,0 +1,70 @@
+/* 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";
+
+const kToolbarName = "test-unregisterArea-areaType";
+const kUnregisterAreaTestWidget = "test-widget-for-unregisterArea-areaType";
+const kTestWidget = "test-widget-no-area-areaType";
+registerCleanupFunction(removeCustomToolbars);
+
+registerCleanupFunction(() => {
+  try {
+    CustomizableUI.destroyWidget(kTestWidget);
+    CustomizableUI.destroyWidget(kUnregisterAreaTestWidget);
+  } catch (ex) {
+    console.error(ex);
+  }
+});
+
+function checkAreaType(widget) {
+  try {
+    // widget.areaType returns either null or undefined
+    ok(!widget.areaType, "areaType should be null");
+  } catch (ex) {
+    info("Fetching areaType threw: " + ex);
+    ok(false, "areaType getter shouldn't throw.");
+  }
+}
+
+// widget wrappers in unregisterArea'd areas and nowhere shouldn't throw when checking areaTypes.
+add_task(async function () {
+  // Using the ID before it's been created will imply a XUL wrapper; we'll test
+  // an API-based wrapper below
+  let toolbarNode = createToolbarWithPlacements(kToolbarName, [
+    kUnregisterAreaTestWidget,
+  ]);
+  CustomizableUI.unregisterArea(kToolbarName);
+  toolbarNode.remove();
+
+  let w = CustomizableUI.getWidget(kUnregisterAreaTestWidget);
+  checkAreaType(w);
+
+  w = CustomizableUI.getWidget(kTestWidget);
+  checkAreaType(w);
+
+  let spec = {
+    id: kUnregisterAreaTestWidget,
+    type: "button",
+    removable: true,
+    label: "areaType test",
+    tooltiptext: "areaType test",
+  };
+  CustomizableUI.createWidget(spec);
+  toolbarNode = createToolbarWithPlacements(kToolbarName, [
+    kUnregisterAreaTestWidget,
+  ]);
+  CustomizableUI.unregisterArea(kToolbarName);
+  toolbarNode.remove();
+  w = CustomizableUI.getWidget(spec.id);
+  checkAreaType(w);
+  CustomizableUI.removeWidgetFromArea(kUnregisterAreaTestWidget);
+  checkAreaType(w);
+  // XXXgijs: ensure cleanup function doesn't barf:
+  gAddedToolbars.delete(kToolbarName);
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_956602_remove_special_widget.js b/browser/components/customizableui/test/browser_956602_remove_special_widget.js
new file mode 100644
index 0000000000..237103b79e
--- /dev/null
+++ b/browser/components/customizableui/test/browser_956602_remove_special_widget.js
@@ -0,0 +1,37 @@
+/* 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";
+
+// Adding a separator and then dragging it out of the navbar shouldn't throw
+add_task(async function () {
+  try {
+    let navbar = document.getElementById("nav-bar");
+    let separatorSelector =
+      "toolbarseparator[id^=customizableui-special-separator]";
+    ok(
+      !navbar.querySelector(separatorSelector),
+      "Shouldn't be a separator in the navbar"
+    );
+    CustomizableUI.addWidgetToArea("separator", "nav-bar");
+    await startCustomizing();
+    let separator = navbar.querySelector(separatorSelector);
+    ok(separator, "There should be a separator in the navbar now.");
+    let palette = document.getElementById("customization-palette");
+    simulateItemDrag(separator, palette);
+    ok(
+      !palette.querySelector(separatorSelector),
+      "No separator in the palette."
+    );
+  } catch (ex) {
+    console.error(ex);
+    ok(false, "Shouldn't throw an exception moving an item to the navbar.");
+  } finally {
+    await endCustomizing();
+  }
+});
+
+add_task(async function asyncCleanup() {
+  resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_962069_drag_to_overflow_chevron.js b/browser/components/customizableui/test/browser_962069_drag_to_overflow_chevron.js
new file mode 100644
index 0000000000..edfc8ff06d
--- /dev/null
+++ b/browser/components/customizableui/test/browser_962069_drag_to_overflow_chevron.js
@@ -0,0 +1,79 @@
+/* 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";
+
+var originalWindowWidth;
+
+// Drag to overflow chevron should open the overflow panel.
+add_task(async function () {
+  // Load a page so the identity box can be dragged.
+  BrowserTestUtils.loadURIString(gBrowser, "http://mochi.test:8888/");
+  await BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+
+  originalWindowWidth = window.outerWidth;
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start with a non-overflowing toolbar."
+  );
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() => navbar.hasAttribute("overflowing"));
+  ok(navbar.hasAttribute("overflowing"), "Should have an overflowing toolbar.");
+
+  let widgetOverflowPanel = document.getElementById("widget-overflow");
+  let panelShownPromise = promisePanelElementShown(window, widgetOverflowPanel);
+  let identityBox = document.getElementById("identity-icon-box");
+  let overflowChevron = document.getElementById("nav-bar-overflow-button");
+
+  // Listen for hiding immediately so we don't miss the event because of the
+  // async-ness of the 'shown' yield...
+  let panelHiddenPromise = promisePanelElementHidden(
+    window,
+    widgetOverflowPanel
+  );
+
+  var ds = Cc["@mozilla.org/widget/dragservice;1"].getService(
+    Ci.nsIDragService
+  );
+
+  ds.startDragSessionForTests(
+    Ci.nsIDragService.DRAGDROP_ACTION_MOVE |
+      Ci.nsIDragService.DRAGDROP_ACTION_COPY |
+      Ci.nsIDragService.DRAGDROP_ACTION_LINK
+  );
+  try {
+    var [result, dataTransfer] = EventUtils.synthesizeDragOver(
+      identityBox,
+      overflowChevron
+    );
+
+    // Wait for showing panel before ending drag session.
+    await panelShownPromise;
+
+    EventUtils.synthesizeDropAfterDragOver(
+      result,
+      dataTransfer,
+      overflowChevron
+    );
+  } finally {
+    ds.endDragSession(true);
+  }
+
+  info("Overflow panel is shown.");
+
+  widgetOverflowPanel.hidePopup();
+  await panelHiddenPromise;
+});
+
+add_task(async function () {
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should not have an overflowing toolbar."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_963639_customizing_attribute_non_customizable_toolbar.js b/browser/components/customizableui/test/browser_963639_customizing_attribute_non_customizable_toolbar.js
new file mode 100644
index 0000000000..db829ab411
--- /dev/null
+++ b/browser/components/customizableui/test/browser_963639_customizing_attribute_non_customizable_toolbar.js
@@ -0,0 +1,48 @@
+/* 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";
+
+const kToolbar = "test-toolbar-963639-non-customizable-customizing-attribute";
+
+add_task(async function () {
+  info(
+    "Test for Bug 963639 - CustomizeMode _onToolbarVisibilityChange sets @customizing on non-customizable toolbars"
+  );
+
+  let toolbar = document.createXULElement("toolbar");
+  toolbar.id = kToolbar;
+  gNavToolbox.appendChild(toolbar);
+
+  let testToolbar = document.getElementById(kToolbar);
+  ok(testToolbar, "Toolbar was created.");
+  is(
+    gNavToolbox.getElementsByAttribute("id", kToolbar).length,
+    1,
+    "Toolbar was added to the navigator toolbox"
+  );
+
+  toolbar.setAttribute(
+    "toolbarname",
+    "NonCustomizableToolbarCustomizingAttribute"
+  );
+  toolbar.setAttribute("collapsed", "true");
+
+  await startCustomizing();
+  window.setToolbarVisibility(toolbar, "true");
+  isnot(
+    toolbar.getAttribute("customizing"),
+    "true",
+    "Toolbar doesn't have the customizing attribute"
+  );
+
+  await endCustomizing();
+  gNavToolbox.removeChild(toolbar);
+
+  is(
+    gNavToolbox.getElementsByAttribute("id", kToolbar).length,
+    0,
+    "Toolbar was removed from the navigator toolbox"
+  );
+});
diff --git a/browser/components/customizableui/test/browser_968565_insert_before_hidden_items.js b/browser/components/customizableui/test/browser_968565_insert_before_hidden_items.js
new file mode 100644
index 0000000000..dbc45880d2
--- /dev/null
+++ b/browser/components/customizableui/test/browser_968565_insert_before_hidden_items.js
@@ -0,0 +1,60 @@
+/* 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";
+
+const kHidden1Id = "test-hidden-button-1";
+const kHidden2Id = "test-hidden-button-2";
+
+var navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+// When we drag an item onto a customizable area, and not over a specific target, we
+// should assume that we're appending them to the area. If doing so, we should scan
+// backwards over any hidden items and insert the item before those hidden items.
+add_task(async function () {
+  ok(CustomizableUI.inDefaultState, "Should be in the default state");
+
+  // Iterate backwards over the items in the nav-bar until we find the first
+  // one that is not hidden.
+  let placements = CustomizableUI.getWidgetsInArea(CustomizableUI.AREA_NAVBAR);
+  let lastVisible = null;
+  for (let widgetGroup of placements.reverse()) {
+    let widget = widgetGroup.forWindow(window);
+    if (widget && widget.node && !widget.node.hidden) {
+      lastVisible = widget.node;
+      break;
+    }
+  }
+
+  if (!lastVisible) {
+    ok(false, "Apparently, there are no visible items in the nav-bar.");
+  }
+
+  info("The last visible item in the nav-bar has ID: " + lastVisible.id);
+
+  let hidden1 = createDummyXULButton(kHidden1Id, "You can't see me");
+  let hidden2 = createDummyXULButton(kHidden2Id, "You can't see me either.");
+  hidden1.hidden = hidden2.hidden = true;
+
+  // Make sure we have some hidden items at the end of the nav-bar.
+  CustomizableUI.addWidgetToArea(kHidden1Id, "nav-bar");
+  CustomizableUI.addWidgetToArea(kHidden2Id, "nav-bar");
+
+  // Drag an item and drop it onto the nav-bar customization target, but
+  // not over a particular item.
+  await startCustomizing();
+  let homeButton = document.getElementById("home-button");
+  let navbarTarget = CustomizableUI.getCustomizationTarget(navbar);
+  simulateItemDrag(homeButton, navbarTarget, "end");
+
+  await endCustomizing();
+
+  is(
+    homeButton.previousElementSibling.id,
+    lastVisible.id,
+    "The downloads button should be placed after the last visible item."
+  );
+
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_969427_recreate_destroyed_widget_after_reset.js b/browser/components/customizableui/test/browser_969427_recreate_destroyed_widget_after_reset.js
new file mode 100644
index 0000000000..8207cd5737
--- /dev/null
+++ b/browser/components/customizableui/test/browser_969427_recreate_destroyed_widget_after_reset.js
@@ -0,0 +1,47 @@
+/* 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";
+
+function getPlacementArea(id) {
+  let placement = CustomizableUI.getPlacementOfWidget(id);
+  return placement && placement.area;
+}
+
+// Check that a destroyed widget recreated after a reset call goes to
+// the navigation bar.
+add_task(function () {
+  const kWidgetId = "test-recreate-after-reset";
+  let spec = {
+    id: kWidgetId,
+    label: "Test re-create after reset.",
+    removable: true,
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  };
+
+  CustomizableUI.createWidget(spec);
+  is(
+    getPlacementArea(kWidgetId),
+    CustomizableUI.AREA_NAVBAR,
+    "widget is in the navigation bar"
+  );
+
+  CustomizableUI.destroyWidget(kWidgetId);
+  isnot(
+    getPlacementArea(kWidgetId),
+    CustomizableUI.AREA_NAVBAR,
+    "widget removed from the navigation bar"
+  );
+
+  CustomizableUI.reset();
+
+  CustomizableUI.createWidget(spec);
+  is(
+    getPlacementArea(kWidgetId),
+    CustomizableUI.AREA_NAVBAR,
+    "widget recreated and added back to the nav bar"
+  );
+
+  CustomizableUI.destroyWidget(kWidgetId);
+});
diff --git a/browser/components/customizableui/test/browser_969661_character_encoding_navbar_disabled.js b/browser/components/customizableui/test/browser_969661_character_encoding_navbar_disabled.js
new file mode 100644
index 0000000000..71e83274c2
--- /dev/null
+++ b/browser/components/customizableui/test/browser_969661_character_encoding_navbar_disabled.js
@@ -0,0 +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";
+
+// Adding the character encoding menu to the panel, exiting customize mode,
+// and moving it to the nav-bar should have it disabled if the page in the
+// content area isn't eligible to have its encoding overridden.
+add_task(async function () {
+  await startCustomizing();
+  CustomizableUI.addWidgetToArea(
+    "characterencoding-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  await endCustomizing();
+  await document.getElementById("nav-bar").overflowable.show();
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  PanelUI.overflowPanel.hidePopup();
+  await panelHiddenPromise;
+  CustomizableUI.addWidgetToArea("characterencoding-button", "nav-bar");
+  let button = document.getElementById("characterencoding-button");
+  ok(button.hasAttribute("disabled"), "Button should be disabled");
+});
+
+add_task(function asyncCleanup() {
+  resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_970511_undo_restore_default.js b/browser/components/customizableui/test/browser_970511_undo_restore_default.js
new file mode 100644
index 0000000000..5477b41b80
--- /dev/null
+++ b/browser/components/customizableui/test/browser_970511_undo_restore_default.js
@@ -0,0 +1,274 @@
+/* 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";
+
+requestLongerTimeout(2);
+
+// Restoring default should reset density and show an "undo" option which undoes
+// the restoring operation.
+add_task(async function () {
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.compactmode.show", true]],
+  });
+  let stopReloadButtonId = "stop-reload-button";
+  CustomizableUI.removeWidgetFromArea(stopReloadButtonId);
+  await startCustomizing();
+  ok(!CustomizableUI.inDefaultState, "Not in default state to begin with");
+  is(
+    CustomizableUI.getPlacementOfWidget(stopReloadButtonId),
+    null,
+    "Stop/reload button is in palette"
+  );
+  let undoResetButton = document.getElementById(
+    "customization-undo-reset-button"
+  );
+  is(undoResetButton.hidden, true, "The undo button is hidden before reset");
+
+  let densityButton = document.getElementById("customization-uidensity-button");
+  let popup = document.getElementById("customization-uidensity-menu");
+  let popupShownPromise = popupShown(popup);
+  EventUtils.synthesizeMouseAtCenter(densityButton, {});
+  info("Clicked on density button");
+  await popupShownPromise;
+
+  let compactModeItem = document.getElementById(
+    "customization-uidensity-menuitem-compact"
+  );
+  let win = document.getElementById("main-window");
+  let densityChangedPromise = new Promise(resolve => {
+    let observer = new MutationObserver(() => {
+      if (win.getAttribute("uidensity") == "compact") {
+        resolve();
+        observer.disconnect();
+      }
+    });
+    observer.observe(win, {
+      attributes: true,
+      attributeFilter: ["uidensity"],
+    });
+  });
+
+  compactModeItem.doCommand();
+  info("Clicked on compact density");
+  await densityChangedPromise;
+
+  await gCustomizeMode.reset();
+
+  ok(CustomizableUI.inDefaultState, "In default state after reset");
+  is(undoResetButton.hidden, false, "The undo button is visible after reset");
+  is(
+    win.hasAttribute("uidensity"),
+    false,
+    "The window has been restored to normal density."
+  );
+
+  await gCustomizeMode.undoReset();
+
+  is(
+    win.getAttribute("uidensity"),
+    "compact",
+    "Density has been reset to compact."
+  );
+  ok(!CustomizableUI.inDefaultState, "Not in default state after undo-reset");
+  is(
+    undoResetButton.hidden,
+    true,
+    "The undo button is hidden after clicking on the undo button"
+  );
+  is(
+    CustomizableUI.getPlacementOfWidget(stopReloadButtonId),
+    null,
+    "Stop/reload button is in palette"
+  );
+
+  await gCustomizeMode.reset();
+  await SpecialPowers.popPrefEnv();
+});
+
+// Performing an action after a reset will hide the undo button.
+add_task(async function action_after_reset_hides_undo() {
+  let stopReloadButtonId = "stop-reload-button";
+  CustomizableUI.removeWidgetFromArea(stopReloadButtonId);
+  ok(!CustomizableUI.inDefaultState, "Not in default state to begin with");
+  is(
+    CustomizableUI.getPlacementOfWidget(stopReloadButtonId),
+    null,
+    "Stop/reload button is in palette"
+  );
+  let undoResetButton = document.getElementById(
+    "customization-undo-reset-button"
+  );
+  is(undoResetButton.hidden, true, "The undo button is hidden before reset");
+
+  await gCustomizeMode.reset();
+
+  ok(CustomizableUI.inDefaultState, "In default state after reset");
+  is(undoResetButton.hidden, false, "The undo button is visible after reset");
+
+  CustomizableUI.addWidgetToArea(
+    stopReloadButtonId,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  is(
+    undoResetButton.hidden,
+    true,
+    "The undo button is hidden after another change"
+  );
+});
+
+// "Restore defaults", exiting customize, and re-entering shouldn't show the Undo button
+add_task(async function () {
+  let undoResetButton = document.getElementById(
+    "customization-undo-reset-button"
+  );
+  is(undoResetButton.hidden, true, "The undo button is hidden before a reset");
+  ok(
+    !CustomizableUI.inDefaultState,
+    "The browser should not be in default state"
+  );
+  await gCustomizeMode.reset();
+
+  is(undoResetButton.hidden, false, "The undo button is visible after a reset");
+  await endCustomizing();
+  await startCustomizing();
+  is(
+    undoResetButton.hidden,
+    true,
+    "The undo reset button should be hidden after entering customization mode"
+  );
+});
+
+// Bug 971626 - Restore Defaults should collapse the Title Bar
+add_task(async function () {
+  {
+    const supported = TabsInTitlebar.systemSupported;
+    is(typeof supported, "boolean");
+    info("TabsInTitlebar support: " + supported);
+    if (!supported) {
+      return;
+    }
+  }
+
+  const kDefaultValue = Services.appinfo.drawInTitlebar;
+  let restoreDefaultsButton = document.getElementById(
+    "customization-reset-button"
+  );
+  let titlebarCheckbox = document.getElementById(
+    "customization-titlebar-visibility-checkbox"
+  );
+  let undoResetButton = document.getElementById(
+    "customization-undo-reset-button"
+  );
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be in default state at start of test"
+  );
+  ok(
+    restoreDefaultsButton.disabled,
+    "Restore defaults button should be disabled when in default state"
+  );
+  is(
+    titlebarCheckbox.hasAttribute("checked"),
+    !kDefaultValue,
+    "Title bar checkbox should reflect pref value"
+  );
+  is(
+    undoResetButton.hidden,
+    true,
+    "Undo reset button should be hidden at start of test"
+  );
+
+  let prefName = "browser.tabs.inTitlebar";
+  Services.prefs.setIntPref(prefName, !kDefaultValue);
+  ok(
+    !restoreDefaultsButton.disabled,
+    "Restore defaults button should be enabled when pref changed"
+  );
+  is(
+    Services.appinfo.drawInTitlebar,
+    !kDefaultValue,
+    "Title bar checkbox should reflect changed pref value"
+  );
+  is(
+    titlebarCheckbox.hasAttribute("checked"),
+    kDefaultValue,
+    "Title bar checkbox should reflect changed pref value"
+  );
+  ok(
+    !CustomizableUI.inDefaultState,
+    "With titlebar flipped, no longer default"
+  );
+  is(
+    undoResetButton.hidden,
+    true,
+    "Undo reset button should be hidden after pref change"
+  );
+
+  await gCustomizeMode.reset();
+  ok(
+    restoreDefaultsButton.disabled,
+    "Restore defaults button should be disabled after reset"
+  );
+  is(
+    titlebarCheckbox.hasAttribute("checked"),
+    !kDefaultValue,
+    "Title bar checkbox should reflect default value after reset"
+  );
+  is(
+    Services.prefs.getIntPref(prefName),
+    2,
+    "Reset should reset drawInTitlebar"
+  );
+  is(
+    Services.appinfo.drawInTitlebar,
+    kDefaultValue,
+    "Default state should be restored"
+  );
+  ok(CustomizableUI.inDefaultState, "In default state after titlebar reset");
+  is(
+    undoResetButton.hidden,
+    false,
+    "Undo reset button should be visible after reset"
+  );
+  ok(
+    !undoResetButton.disabled,
+    "Undo reset button should be enabled after reset"
+  );
+
+  await gCustomizeMode.undoReset();
+  ok(
+    !restoreDefaultsButton.disabled,
+    "Restore defaults button should be enabled after undo-reset"
+  );
+  is(
+    titlebarCheckbox.hasAttribute("checked"),
+    kDefaultValue,
+    "Title bar checkbox should reflect undo-reset value"
+  );
+  ok(!CustomizableUI.inDefaultState, "No longer in default state after undo");
+  is(
+    Services.prefs.getIntPref(prefName),
+    kDefaultValue ? 0 : 1,
+    "Undo-reset goes back to previous pref value"
+  );
+  is(
+    undoResetButton.hidden,
+    true,
+    "Undo reset button should be hidden after undo-reset clicked"
+  );
+
+  Services.prefs.clearUserPref(prefName);
+  ok(CustomizableUI.inDefaultState, "In default state after pref cleared");
+  is(
+    undoResetButton.hidden,
+    true,
+    "Undo reset button should be hidden at end of test"
+  );
+});
+
+add_task(async function asyncCleanup() {
+  await gCustomizeMode.reset();
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_972267_customizationchange_events.js b/browser/components/customizableui/test/browser_972267_customizationchange_events.js
new file mode 100644
index 0000000000..7d27b94136
--- /dev/null
+++ b/browser/components/customizableui/test/browser_972267_customizationchange_events.js
@@ -0,0 +1,39 @@
+/* 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";
+
+// Create a new window, then move the stop/reload button to the menu and check both windows have
+// customizationchange events fire on the toolbox:
+add_task(async function () {
+  let newWindow = await openAndLoadWindow();
+  let otherToolbox = newWindow.gNavToolbox;
+
+  let handlerCalledCount = 0;
+  let handler = ev => {
+    handlerCalledCount++;
+  };
+
+  let stopReloadButton = document.getElementById("stop-reload-button");
+
+  gNavToolbox.addEventListener("customizationchange", handler);
+  otherToolbox.addEventListener("customizationchange", handler);
+
+  await gCustomizeMode.addToPanel(stopReloadButton);
+
+  is(handlerCalledCount, 2, "Should be called for both windows.");
+
+  handlerCalledCount = 0;
+  gCustomizeMode.addToToolbar(stopReloadButton);
+  is(handlerCalledCount, 2, "Should be called for both windows.");
+
+  gNavToolbox.removeEventListener("customizationchange", handler);
+  otherToolbox.removeEventListener("customizationchange", handler);
+
+  await promiseWindowClosed(newWindow);
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_976792_insertNodeInWindow.js b/browser/components/customizableui/test/browser_976792_insertNodeInWindow.js
new file mode 100644
index 0000000000..6a8cb26958
--- /dev/null
+++ b/browser/components/customizableui/test/browser_976792_insertNodeInWindow.js
@@ -0,0 +1,597 @@
+/* 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";
+
+const kToolbarName = "test-insertNodeInWindow-placements-toolbar";
+const kTestWidgetPrefix = "test-widget-for-insertNodeInWindow-placements-";
+
+/*
+Tries to replicate the situation of having a placement list like this:
+
+exists-1,trying-to-insert-this,doesn't-exist,exists-2
+*/
+add_task(async function () {
+  let testWidgetExists = [true, false, false, true];
+  let widgetIds = [];
+  for (let i = 0; i < testWidgetExists.length; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    if (testWidgetExists[i]) {
+      let spec = {
+        id,
+        type: "button",
+        removable: true,
+        label: "test",
+        tooltiptext: "" + i,
+      };
+      CustomizableUI.createWidget(spec);
+    }
+  }
+
+  let toolbarNode = createToolbarWithPlacements(kToolbarName, widgetIds);
+  assertAreaPlacements(kToolbarName, widgetIds);
+
+  let btnId = kTestWidgetPrefix + 1;
+  let btn = createDummyXULButton(btnId, "test");
+  CustomizableUI.ensureWidgetPlacedInWindow(btnId, window);
+
+  is(
+    btn.parentNode.id,
+    kToolbarName,
+    "New XUL widget should be placed inside new toolbar"
+  );
+
+  is(
+    btn.previousElementSibling.id,
+    toolbarNode.firstElementChild.id,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  btn.remove();
+  removeCustomToolbars();
+  await resetCustomization();
+});
+
+/*
+Tests nodes get placed inside the toolbar's overflow as expected. Replicates a
+situation similar to:
+
+exists-1,exists-2,overflow-1,trying-to-insert-this,overflow-2
+*/
+add_task(async function () {
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+  let widgetIds = [];
+  for (let i = 0; i < 5; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    let spec = {
+      id,
+      type: "button",
+      removable: true,
+      label: "insertNodeInWindow test",
+      tooltiptext: "" + i,
+    };
+    CustomizableUI.createWidget(spec);
+    CustomizableUI.addWidgetToArea(id, "nav-bar");
+  }
+
+  for (let id of widgetIds) {
+    document.getElementById(id).style.minWidth = "200px";
+  }
+
+  let originalWindowWidth = window.outerWidth;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () =>
+      navbar.hasAttribute("overflowing") &&
+      !navbar.querySelector("#" + widgetIds[0])
+  );
+
+  let testWidgetId = kTestWidgetPrefix + 3;
+
+  CustomizableUI.destroyWidget(testWidgetId);
+
+  let btn = createDummyXULButton(testWidgetId, "test");
+  CustomizableUI.ensureWidgetPlacedInWindow(testWidgetId, window);
+
+  ok(
+    navbar.overflowable.isInOverflowList(btn),
+    "New XUL widget should be placed inside overflow of toolbar"
+  );
+  is(
+    btn.previousElementSibling.id,
+    kTestWidgetPrefix + 2,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+  is(
+    btn.nextElementSibling.id,
+    kTestWidgetPrefix + 4,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  CustomizableUI.removeWidgetFromArea(btn.id, kToolbarName);
+  btn.remove();
+  await resetCustomization();
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+/*
+Tests nodes get placed inside the toolbar's overflow as expected. Replicates a
+placements situation similar to:
+
+exists-1,exists-2,overflow-1,doesn't-exist,trying-to-insert-this,overflow-2
+*/
+add_task(async function () {
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+  let widgetIds = [];
+  for (let i = 0; i < 5; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    let spec = {
+      id,
+      type: "button",
+      removable: true,
+      label: "insertNodeInWindow test",
+      tooltiptext: "" + i,
+    };
+    CustomizableUI.createWidget(spec);
+    CustomizableUI.addWidgetToArea(id, "nav-bar");
+  }
+
+  for (let id of widgetIds) {
+    document.getElementById(id).style.minWidth = "200px";
+  }
+
+  let originalWindowWidth = window.outerWidth;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () =>
+      navbar.hasAttribute("overflowing") &&
+      !navbar.querySelector("#" + widgetIds[0])
+  );
+
+  let testWidgetId = kTestWidgetPrefix + 3;
+
+  CustomizableUI.destroyWidget(kTestWidgetPrefix + 2);
+  CustomizableUI.destroyWidget(testWidgetId);
+
+  let btn = createDummyXULButton(testWidgetId, "test");
+  CustomizableUI.ensureWidgetPlacedInWindow(testWidgetId, window);
+
+  ok(
+    navbar.overflowable.isInOverflowList(btn),
+    "New XUL widget should be placed inside overflow of toolbar"
+  );
+  is(
+    btn.previousElementSibling.id,
+    kTestWidgetPrefix + 1,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+  is(
+    btn.nextElementSibling.id,
+    kTestWidgetPrefix + 4,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  CustomizableUI.removeWidgetFromArea(btn.id, kToolbarName);
+  btn.remove();
+  await resetCustomization();
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+/*
+Tests nodes get placed inside the toolbar's overflow as expected. Replicates a
+placements situation similar to:
+
+exists-1,exists-2,overflow-1,doesn't-exist,trying-to-insert-this,doesn't-exist
+*/
+add_task(async function () {
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+  let widgetIds = [];
+  for (let i = 0; i < 5; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    let spec = {
+      id,
+      type: "button",
+      removable: true,
+      label: "insertNodeInWindow test",
+      tooltiptext: "" + i,
+    };
+    CustomizableUI.createWidget(spec);
+    CustomizableUI.addWidgetToArea(id, "nav-bar");
+  }
+
+  for (let id of widgetIds) {
+    document.getElementById(id).style.minWidth = "200px";
+  }
+
+  let originalWindowWidth = window.outerWidth;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () =>
+      navbar.hasAttribute("overflowing") &&
+      !navbar.querySelector("#" + widgetIds[0])
+  );
+
+  let testWidgetId = kTestWidgetPrefix + 3;
+
+  CustomizableUI.destroyWidget(kTestWidgetPrefix + 2);
+  CustomizableUI.destroyWidget(testWidgetId);
+  CustomizableUI.destroyWidget(kTestWidgetPrefix + 4);
+
+  let btn = createDummyXULButton(testWidgetId, "test");
+  CustomizableUI.ensureWidgetPlacedInWindow(testWidgetId, window);
+
+  ok(
+    navbar.overflowable.isInOverflowList(btn),
+    "New XUL widget should be placed inside overflow of toolbar"
+  );
+  is(
+    btn.previousElementSibling.id,
+    kTestWidgetPrefix + 1,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+  is(
+    btn.nextElementSibling,
+    null,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  CustomizableUI.removeWidgetFromArea(btn.id, kToolbarName);
+  btn.remove();
+  await resetCustomization();
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+/*
+Tests nodes get placed inside the toolbar's overflow as expected. Replicates a
+placements situation similar to:
+
+exists-1,exists-2,overflow-1,can't-overflow,trying-to-insert-this,overflow-2
+*/
+add_task(async function () {
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+  let widgetIds = [];
+  for (let i = 5; i >= 0; i--) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    let spec = {
+      id,
+      type: "button",
+      removable: true,
+      label: "insertNodeInWindow test",
+      tooltiptext: "" + i,
+    };
+    CustomizableUI.createWidget(spec);
+    CustomizableUI.addWidgetToArea(id, "nav-bar", 0);
+  }
+
+  for (let i = 10; i < 15; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    let spec = {
+      id,
+      type: "button",
+      removable: true,
+      label: "insertNodeInWindow test",
+      tooltiptext: "" + i,
+    };
+    CustomizableUI.createWidget(spec);
+    CustomizableUI.addWidgetToArea(id, "nav-bar");
+  }
+
+  for (let id of widgetIds) {
+    document.getElementById(id).style.minWidth = "200px";
+  }
+
+  let originalWindowWidth = window.outerWidth;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  // Wait for all the widgets to overflow. We can't just wait for the
+  // `overflowing` attribute because we leave time for layout flushes
+  // inbetween, so it's possible for the timeout to run before the
+  // navbar has "settled"
+  await TestUtils.waitForCondition(() => {
+    return (
+      navbar.hasAttribute("overflowing") &&
+      CustomizableUI.getCustomizationTarget(
+        navbar
+      ).lastElementChild.getAttribute("overflows") == "false"
+    );
+  });
+
+  // Find last widget that doesn't allow overflowing
+  let nonOverflowing =
+    CustomizableUI.getCustomizationTarget(navbar).lastElementChild;
+  is(
+    nonOverflowing.getAttribute("overflows"),
+    "false",
+    "Last child is expected to not allow overflowing"
+  );
+  isnot(
+    nonOverflowing.getAttribute("skipintoolbarset"),
+    "true",
+    "Last child is expected to not be skipintoolbarset"
+  );
+
+  let testWidgetId = kTestWidgetPrefix + 10;
+  CustomizableUI.destroyWidget(testWidgetId);
+
+  let btn = createDummyXULButton(testWidgetId, "test");
+  CustomizableUI.ensureWidgetPlacedInWindow(testWidgetId, window);
+
+  ok(
+    navbar.overflowable.isInOverflowList(btn),
+    "New XUL widget should be placed inside overflow of toolbar"
+  );
+  is(
+    btn.nextElementSibling.id,
+    kTestWidgetPrefix + 11,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  CustomizableUI.removeWidgetFromArea(btn.id, kToolbarName);
+  btn.remove();
+  await resetCustomization();
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+});
+
+/*
+Tests nodes get placed inside the toolbar's overflow as expected. Replicates a
+placements situation similar to:
+
+exists-1,exists-2,overflow-1,trying-to-insert-this,can't-overflow,overflow-2
+*/
+add_task(async function () {
+  let widgetIds = [];
+  let missingId = 2;
+  let nonOverflowableId = 3;
+  for (let i = 0; i < 5; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    if (i != missingId) {
+      // Setting min-width to make the overflow state not depend on styling of the button and/or
+      // screen width
+      let spec = {
+        id,
+        type: "button",
+        removable: true,
+        label: "test",
+        tooltiptext: "" + i,
+        onCreated(node) {
+          node.style.minWidth = "200px";
+          if (id == kTestWidgetPrefix + nonOverflowableId) {
+            node.setAttribute("overflows", false);
+          }
+        },
+      };
+      info("Creating: " + id);
+      CustomizableUI.createWidget(spec);
+    }
+  }
+
+  let toolbarNode = createOverflowableToolbarWithPlacements(
+    kToolbarName,
+    widgetIds
+  );
+  assertAreaPlacements(kToolbarName, widgetIds);
+  ok(
+    !toolbarNode.hasAttribute("overflowing"),
+    "Toolbar shouldn't overflow to start with."
+  );
+
+  let originalWindowWidth = window.outerWidth;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () =>
+      toolbarNode.hasAttribute("overflowing") &&
+      !toolbarNode.querySelector("#" + widgetIds[1])
+  );
+  ok(
+    toolbarNode.hasAttribute("overflowing"),
+    "Should have an overflowing toolbar."
+  );
+
+  let btnId = kTestWidgetPrefix + missingId;
+  let btn = createDummyXULButton(btnId, "test");
+  CustomizableUI.ensureWidgetPlacedInWindow(btnId, window);
+
+  is(
+    btn.parentNode.id,
+    kToolbarName + "-overflow-list",
+    "New XUL widget should be placed inside new toolbar's overflow"
+  );
+  is(
+    btn.previousElementSibling.id,
+    kTestWidgetPrefix + 1,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+  is(
+    btn.nextElementSibling.id,
+    kTestWidgetPrefix + 4,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () => !toolbarNode.hasAttribute("overflowing")
+  );
+
+  btn.remove();
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  removeCustomToolbars();
+  await resetCustomization();
+});
+
+/*
+Tests nodes do *not* get placed in the toolbar's overflow. Replicates a
+plcements situation similar to:
+
+exists-1,trying-to-insert-this,exists-2,overflowed-1
+*/
+add_task(async function () {
+  let widgetIds = [];
+  let missingId = 1;
+  for (let i = 0; i < 5; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    if (i != missingId) {
+      // Setting min-width to make the overflow state not depend on styling of the button and/or
+      // screen width
+      let spec = {
+        id,
+        type: "button",
+        removable: true,
+        label: "test",
+        tooltiptext: "" + i,
+        onCreated(node) {
+          node.style.minWidth = "200px";
+        },
+      };
+      info("Creating: " + id);
+      CustomizableUI.createWidget(spec);
+    }
+  }
+
+  let toolbarNode = createOverflowableToolbarWithPlacements(
+    kToolbarName,
+    widgetIds
+  );
+  assertAreaPlacements(kToolbarName, widgetIds);
+  ok(
+    !toolbarNode.hasAttribute("overflowing"),
+    "Toolbar shouldn't overflow to start with."
+  );
+
+  let originalWindowWidth = window.outerWidth;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() =>
+    toolbarNode.hasAttribute("overflowing")
+  );
+  ok(
+    toolbarNode.hasAttribute("overflowing"),
+    "Should have an overflowing toolbar."
+  );
+
+  let btnId = kTestWidgetPrefix + missingId;
+  let btn = createDummyXULButton(btnId, "test");
+  CustomizableUI.ensureWidgetPlacedInWindow(btnId, window);
+
+  is(
+    btn.parentNode.id,
+    kToolbarName + "-target",
+    "New XUL widget should be placed inside new toolbar"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () => !toolbarNode.hasAttribute("overflowing")
+  );
+
+  btn.remove();
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  removeCustomToolbars();
+  await resetCustomization();
+});
+
+/*
+Tests inserting a node onto the end of an overflowing toolbar *doesn't* put it in
+the overflow list when the widget disallows overflowing. ie:
+
+exists-1,exists-2,overflows-1,trying-to-insert-this
+
+Where trying-to-insert-this has overflows=false
+*/
+add_task(async function () {
+  let widgetIds = [];
+  let missingId = 3;
+  for (let i = 0; i < 5; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    if (i != missingId) {
+      // Setting min-width to make the overflow state not depend on styling of the button and/or
+      // screen width
+      let spec = {
+        id,
+        type: "button",
+        removable: true,
+        label: "test",
+        tooltiptext: "" + i,
+        onCreated(node) {
+          node.style.minWidth = "200px";
+        },
+      };
+      info("Creating: " + id);
+      CustomizableUI.createWidget(spec);
+    }
+  }
+
+  let toolbarNode = createOverflowableToolbarWithPlacements(
+    kToolbarName,
+    widgetIds
+  );
+  assertAreaPlacements(kToolbarName, widgetIds);
+  ok(
+    !toolbarNode.hasAttribute("overflowing"),
+    "Toolbar shouldn't overflow to start with."
+  );
+
+  let originalWindowWidth = window.outerWidth;
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() =>
+    toolbarNode.hasAttribute("overflowing")
+  );
+  ok(
+    toolbarNode.hasAttribute("overflowing"),
+    "Should have an overflowing toolbar."
+  );
+
+  let btnId = kTestWidgetPrefix + missingId;
+  let btn = createDummyXULButton(btnId, "test");
+  btn.setAttribute("overflows", false);
+  CustomizableUI.ensureWidgetPlacedInWindow(btnId, window);
+
+  is(
+    btn.parentNode.id,
+    kToolbarName + "-target",
+    "New XUL widget should be placed inside new toolbar"
+  );
+  is(
+    btn.nextElementSibling,
+    null,
+    "insertNodeInWindow should have placed new XUL widget in correct place in DOM according to placements"
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () => !toolbarNode.hasAttribute("overflowing")
+  );
+
+  btn.remove();
+  widgetIds.forEach(id => CustomizableUI.destroyWidget(id));
+  removeCustomToolbars();
+  await resetCustomization();
+});
+
+add_task(async function asyncCleanUp() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_978084_dragEnd_after_move.js b/browser/components/customizableui/test/browser_978084_dragEnd_after_move.js
new file mode 100644
index 0000000000..c818a1b468
--- /dev/null
+++ b/browser/components/customizableui/test/browser_978084_dragEnd_after_move.js
@@ -0,0 +1,52 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+var draggedItem;
+
+/**
+ * Check that customizing-movingItem gets removed on a drop when the item is moved.
+ */
+
+// Drop on the palette
+add_task(async function () {
+  draggedItem = document.createXULElement("toolbarbutton");
+  draggedItem.id = "test-dragEnd-after-move1";
+  draggedItem.setAttribute("label", "Test");
+  draggedItem.setAttribute("removable", "true");
+  let navbar = document.getElementById("nav-bar");
+  CustomizableUI.getCustomizationTarget(navbar).appendChild(draggedItem);
+  await startCustomizing();
+  simulateItemDrag(draggedItem, gCustomizeMode.visiblePalette);
+  is(
+    document.documentElement.hasAttribute("customizing-movingItem"),
+    false,
+    "Make sure customizing-movingItem is removed after dragging to the palette"
+  );
+  await endCustomizing();
+});
+
+// Drop on a customization target itself
+add_task(async function () {
+  draggedItem = document.createXULElement("toolbarbutton");
+  draggedItem.id = "test-dragEnd-after-move2";
+  draggedItem.setAttribute("label", "Test");
+  draggedItem.setAttribute("removable", "true");
+  let dest = createToolbarWithPlacements("test-dragEnd");
+  let navbar = document.getElementById("nav-bar");
+  CustomizableUI.getCustomizationTarget(navbar).appendChild(draggedItem);
+  await startCustomizing();
+  simulateItemDrag(draggedItem, CustomizableUI.getCustomizationTarget(dest));
+  is(
+    document.documentElement.hasAttribute("customizing-movingItem"),
+    false,
+    "Make sure customizing-movingItem is removed"
+  );
+  await endCustomizing();
+});
+
+registerCleanupFunction(async function asyncCleanup() {
+  await endCustomizing();
+  removeCustomToolbars();
+});
diff --git a/browser/components/customizableui/test/browser_980155_add_overflow_toolbar.js b/browser/components/customizableui/test/browser_980155_add_overflow_toolbar.js
new file mode 100644
index 0000000000..7cb78a2ea4
--- /dev/null
+++ b/browser/components/customizableui/test/browser_980155_add_overflow_toolbar.js
@@ -0,0 +1,96 @@
+/* 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";
+
+const kToolbarName = "test-new-overflowable-toolbar";
+const kTestWidgetPrefix = "test-widget-for-overflowable-toolbar-";
+
+add_task(async function addOverflowingToolbar() {
+  let originalWindowWidth = window.outerWidth;
+
+  let widgetIds = [];
+  registerCleanupFunction(() => {
+    try {
+      for (let id of widgetIds) {
+        CustomizableUI.destroyWidget(id);
+      }
+    } catch (ex) {
+      console.error(ex);
+    }
+  });
+
+  for (let i = 0; i < 10; i++) {
+    let id = kTestWidgetPrefix + i;
+    widgetIds.push(id);
+    let spec = {
+      id,
+      type: "button",
+      removable: true,
+      label: "test",
+      tooltiptext: "" + i,
+    };
+    CustomizableUI.createWidget(spec);
+  }
+
+  let toolbarNode = createOverflowableToolbarWithPlacements(
+    kToolbarName,
+    widgetIds
+  );
+  assertAreaPlacements(kToolbarName, widgetIds);
+
+  for (let id of widgetIds) {
+    document.getElementById(id).style.minWidth = "200px";
+  }
+
+  isnot(
+    toolbarNode.overflowable,
+    null,
+    "Toolbar should have overflowable controller"
+  );
+  isnot(
+    CustomizableUI.getCustomizationTarget(toolbarNode),
+    null,
+    "Toolbar should have customization target"
+  );
+  isnot(
+    CustomizableUI.getCustomizationTarget(toolbarNode),
+    toolbarNode,
+    "Customization target should not be toolbar node"
+  );
+
+  let oldChildCount =
+    CustomizableUI.getCustomizationTarget(toolbarNode).childElementCount;
+  let overflowableList = document.getElementById(
+    kToolbarName + "-overflow-list"
+  );
+  let oldOverflowCount = overflowableList.childElementCount;
+
+  isnot(oldChildCount, 0, "Toolbar should have non-overflowing widgets");
+
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(() =>
+    toolbarNode.hasAttribute("overflowing")
+  );
+  ok(
+    toolbarNode.hasAttribute("overflowing"),
+    "Should have an overflowing toolbar."
+  );
+  ok(
+    CustomizableUI.getCustomizationTarget(toolbarNode).childElementCount <
+      oldChildCount,
+    "Should have fewer children."
+  );
+  ok(
+    overflowableList.childElementCount > oldOverflowCount,
+    "Should have more overflowed widgets."
+  );
+
+  window.resizeTo(originalWindowWidth, window.outerHeight);
+});
+
+add_task(async function asyncCleanup() {
+  removeCustomToolbars();
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_981305_separator_insertion.js b/browser/components/customizableui/test/browser_981305_separator_insertion.js
new file mode 100644
index 0000000000..cce18f33a2
--- /dev/null
+++ b/browser/components/customizableui/test/browser_981305_separator_insertion.js
@@ -0,0 +1,89 @@
+/* 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";
+
+var tempElements = [];
+
+function insertTempItemsIntoMenu(parentMenu) {
+  // Last element is null to insert at the end:
+  let beforeEls = [
+    parentMenu.firstElementChild,
+    parentMenu.lastElementChild,
+    null,
+  ];
+  for (let i = 0; i < beforeEls.length; i++) {
+    let sep = document.createXULElement("menuseparator");
+    tempElements.push(sep);
+    parentMenu.insertBefore(sep, beforeEls[i]);
+    let menu = document.createXULElement("menu");
+    tempElements.push(menu);
+    parentMenu.insertBefore(menu, beforeEls[i]);
+    // And another separator for good measure:
+    sep = document.createXULElement("menuseparator");
+    tempElements.push(sep);
+    parentMenu.insertBefore(sep, beforeEls[i]);
+  }
+}
+
+async function checkSeparatorInsertion(menuId, buttonId, subviewId) {
+  info("Checking for duplicate separators in " + buttonId + " widget");
+  let menu = document.getElementById(menuId);
+  insertTempItemsIntoMenu(menu);
+
+  CustomizableUI.addWidgetToArea(
+    buttonId,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  await document.getElementById("nav-bar").overflowable.show();
+
+  let button = document.getElementById(buttonId);
+  button.click();
+  let subview = document.getElementById(subviewId);
+  await BrowserTestUtils.waitForEvent(subview, "ViewShown");
+
+  let subviewBody = subview.firstElementChild;
+  ok(subviewBody.firstElementChild, "Subview should have a kid");
+  is(
+    subviewBody.firstElementChild.localName,
+    "toolbarbutton",
+    "There should be no separators to start with"
+  );
+
+  for (let kid of subviewBody.children) {
+    if (kid.localName == "menuseparator") {
+      ok(
+        kid.previousElementSibling &&
+          kid.previousElementSibling.localName != "menuseparator",
+        "Separators should never have another separator next to them, and should never be the first node."
+      );
+    }
+  }
+
+  let panelHiddenPromise = promiseOverflowHidden(window);
+  PanelUI.overflowPanel.hidePopup();
+  await panelHiddenPromise;
+
+  CustomizableUI.reset();
+}
+
+add_task(async function check_devtools_separator() {
+  const panelviewId = "PanelUI-developer-tools";
+
+  await checkSeparatorInsertion(
+    "menuWebDeveloperPopup",
+    "developer-button",
+    panelviewId
+  );
+});
+
+registerCleanupFunction(function () {
+  for (let el of tempElements) {
+    el.remove();
+  }
+  tempElements = null;
+});
diff --git a/browser/components/customizableui/test/browser_981418-widget-onbeforecreated-handler.js b/browser/components/customizableui/test/browser_981418-widget-onbeforecreated-handler.js
new file mode 100644
index 0000000000..c3aee43404
--- /dev/null
+++ b/browser/components/customizableui/test/browser_981418-widget-onbeforecreated-handler.js
@@ -0,0 +1,65 @@
+/* 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/. */
+/* eslint-disable mozilla/no-arbitrary-setTimeout */
+
+"use strict";
+const kWidgetId = "test-981418-widget-onbeforecreated";
+
+// Should be able to add broken view widget
+add_task(async function testAddOnBeforeCreatedWidget() {
+  let onBeforeCreatedCalled = false;
+  let widgetSpec = {
+    id: kWidgetId,
+    type: "view",
+    viewId: kWidgetId + "idontexistyet",
+    onBeforeCreated(doc) {
+      let view = doc.createXULElement("panelview");
+      view.id = kWidgetId + "idontexistyet";
+      document.getElementById("appMenu-viewCache").appendChild(view);
+      onBeforeCreatedCalled = true;
+    },
+  };
+
+  CustomizableUI.createWidget(widgetSpec);
+  CustomizableUI.addWidgetToArea(kWidgetId, CustomizableUI.AREA_NAVBAR);
+
+  ok(onBeforeCreatedCalled, "onBeforeCreated should have been called");
+
+  let widgetNode = document.getElementById(kWidgetId);
+  let viewNode = document.getElementById(kWidgetId + "idontexistyet");
+  ok(widgetNode, "Widget should exist");
+  ok(viewNode, "Panelview should exist");
+
+  let viewShownPromise = BrowserTestUtils.waitForEvent(viewNode, "ViewShown");
+  widgetNode.click();
+  await viewShownPromise;
+
+  let widgetPanel = document.getElementById("customizationui-widget-panel");
+  ok(widgetPanel, "Widget panel should exist");
+
+  let panelHiddenPromise = promisePanelElementHidden(window, widgetPanel);
+  widgetPanel.hidePopup();
+  await panelHiddenPromise;
+
+  CustomizableUI.addWidgetToArea(
+    kWidgetId,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  await waitForOverflowButtonShown();
+  await document.getElementById("nav-bar").overflowable.show();
+
+  viewShownPromise = BrowserTestUtils.waitForEvent(viewNode, "ViewShown");
+  widgetNode.click();
+  await viewShownPromise;
+
+  let panelHidden = promiseOverflowHidden(window);
+  PanelUI.overflowPanel.hidePopup();
+  await panelHidden;
+
+  CustomizableUI.destroyWidget(kWidgetId);
+});
+
+add_task(async function asyncCleanup() {
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_982656_restore_defaults_builtin_widgets.js b/browser/components/customizableui/test/browser_982656_restore_defaults_builtin_widgets.js
new file mode 100644
index 0000000000..20314d6790
--- /dev/null
+++ b/browser/components/customizableui/test/browser_982656_restore_defaults_builtin_widgets.js
@@ -0,0 +1,82 @@
+/* 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";
+
+// Restoring default should not place addon widgets back in the toolbar
+add_task(async function () {
+  ok(CustomizableUI.inDefaultState, "Default state to begin");
+
+  const kWidgetId =
+    "bug982656-add-on-widget-should-not-restore-to-default-area";
+  let widgetSpec = {
+    id: kWidgetId,
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  };
+  CustomizableUI.createWidget(widgetSpec);
+
+  ok(!CustomizableUI.inDefaultState, "Not in default state after widget added");
+  is(
+    CustomizableUI.getPlacementOfWidget(kWidgetId).area,
+    CustomizableUI.AREA_NAVBAR,
+    "Widget should be in navbar"
+  );
+
+  await resetCustomization();
+
+  ok(CustomizableUI.inDefaultState, "Back in default state after reset");
+  is(
+    CustomizableUI.getPlacementOfWidget(kWidgetId),
+    null,
+    "Widget now in palette"
+  );
+  CustomizableUI.destroyWidget(kWidgetId);
+});
+
+// resetCustomization shouldn't move 3rd party widgets out of custom toolbars
+add_task(async function () {
+  const kToolbarId = "bug982656-toolbar-with-defaultset";
+  const kWidgetId =
+    "bug982656-add-on-widget-should-restore-to-default-area-when-area-is-not-builtin";
+  ok(
+    CustomizableUI.inDefaultState,
+    "Everything should be in its default state."
+  );
+  let toolbar = createToolbarWithPlacements(kToolbarId);
+  ok(CustomizableUI.areas.includes(kToolbarId), "Toolbar has been registered.");
+  is(
+    CustomizableUI.getAreaType(kToolbarId),
+    CustomizableUI.TYPE_TOOLBAR,
+    "Area should be registered as toolbar"
+  );
+
+  let widgetSpec = {
+    id: kWidgetId,
+    defaultArea: kToolbarId,
+  };
+  CustomizableUI.createWidget(widgetSpec);
+
+  ok(
+    !CustomizableUI.inDefaultState,
+    "No longer in default state after toolbar is registered and visible."
+  );
+  is(
+    CustomizableUI.getPlacementOfWidget(kWidgetId).area,
+    kToolbarId,
+    "Widget should be in custom toolbar"
+  );
+
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "Back in default state after reset");
+  is(
+    CustomizableUI.getPlacementOfWidget(kWidgetId).area,
+    kToolbarId,
+    "Widget still in custom toolbar"
+  );
+  ok(toolbar.collapsed, "Custom toolbar should be collapsed after reset");
+
+  toolbar.remove();
+  CustomizableUI.destroyWidget(kWidgetId);
+  CustomizableUI.unregisterArea(kToolbarId);
+});
diff --git a/browser/components/customizableui/test/browser_984455_bookmarks_items_reparenting.js b/browser/components/customizableui/test/browser_984455_bookmarks_items_reparenting.js
new file mode 100644
index 0000000000..6d2295cc16
--- /dev/null
+++ b/browser/components/customizableui/test/browser_984455_bookmarks_items_reparenting.js
@@ -0,0 +1,302 @@
+/* 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";
+
+var gNavBar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+var gOverflowList = document.getElementById(
+  gNavBar.getAttribute("default-overflowtarget")
+);
+
+const kBookmarksButton = "bookmarks-menu-button";
+const kBookmarksItems = "personal-bookmarks";
+const kOriginalWindowWidth = window.outerWidth;
+
+/**
+ * Helper function that opens the bookmarks menu, and returns a Promise that
+ * resolves as soon as the menu is ready for interaction.
+ */
+function bookmarksMenuPanelShown() {
+  return new Promise(resolve => {
+    let bookmarksMenuPopup = document.getElementById("BMB_bookmarksPopup");
+    let onPopupShown = e => {
+      if (e.target == bookmarksMenuPopup) {
+        bookmarksMenuPopup.removeEventListener("popupshown", onPopupShown);
+        resolve();
+      }
+    };
+    bookmarksMenuPopup.addEventListener("popupshown", onPopupShown);
+  });
+}
+
+/**
+ * Checks that the placesContext menu is correctly attached to the
+ * controller of some view. Returns a Promise that resolves as soon
+ * as the context menu is closed.
+ *
+ * @param aItemWithContextMenu the item that we need to synthesize the
+ *        right click on in order to open the context menu.
+ */
+function checkPlacesContextMenu(aItemWithContextMenu) {
+  return (async function () {
+    let contextMenu = document.getElementById("placesContext");
+    let newBookmarkItem = document.getElementById("placesContext_new:bookmark");
+    info("Waiting for context menu on " + aItemWithContextMenu.id);
+    let shownPromise = popupShown(contextMenu);
+    EventUtils.synthesizeMouseAtCenter(aItemWithContextMenu, {
+      type: "contextmenu",
+      button: 2,
+    });
+    await shownPromise;
+
+    ok(
+      !newBookmarkItem.hasAttribute("disabled"),
+      "New bookmark item shouldn't be disabled"
+    );
+
+    info("Closing context menu");
+    let hiddenPromise = popupHidden(contextMenu);
+    // Use hidePopup instead of the closePopup helper because macOS native
+    // context menus can't be closed by synthesized ESC in automation.
+    contextMenu.hidePopup();
+    await hiddenPromise;
+  })();
+}
+
+/**
+ * Opens the bookmarks menu panel, and then opens each of the "special"
+ * submenus in that list. Then it checks that those submenu's context menus
+ * are properly hooked up to a controller.
+ */
+function checkSpecialContextMenus() {
+  return (async function () {
+    let bookmarksMenuButton = document.getElementById(kBookmarksButton);
+    let bookmarksMenuPopup = document.getElementById("BMB_bookmarksPopup");
+
+    const kSpecialItemIDs = {
+      BMB_bookmarksToolbar: "BMB_bookmarksToolbarPopup",
+      BMB_unsortedBookmarks: "BMB_unsortedBookmarksPopup",
+    };
+
+    // Open the bookmarks menu button context menus and ensure that
+    // they have the proper views attached.
+    let shownPromise = bookmarksMenuPanelShown();
+
+    EventUtils.synthesizeMouseAtCenter(bookmarksMenuButton, {});
+    info("Waiting for bookmarks menu popup to show after clicking dropmarker.");
+    await shownPromise;
+
+    for (let menuID in kSpecialItemIDs) {
+      let menuItem = document.getElementById(menuID);
+      let menuPopup = document.getElementById(kSpecialItemIDs[menuID]);
+      info("Waiting to open menu for " + menuID);
+      shownPromise = popupShown(menuPopup);
+      menuPopup.openPopup(menuItem, null, 0, 0, false, false, null);
+      await shownPromise;
+
+      await checkPlacesContextMenu(menuPopup);
+      info("Closing menu for " + menuID);
+      await closePopup(menuPopup);
+    }
+
+    info("Closing bookmarks menu");
+    await closePopup(bookmarksMenuPopup);
+  })();
+}
+
+/**
+ * Closes a focused popup by simulating pressing the Escape key,
+ * and returns a Promise that resolves as soon as the popup is closed.
+ *
+ * @param aPopup the popup node to close.
+ */
+function closePopup(aPopup) {
+  let hiddenPromise = popupHidden(aPopup);
+  EventUtils.synthesizeKey("KEY_Escape");
+  return hiddenPromise;
+}
+
+/**
+ * Helper function that checks that the context menu of the
+ * bookmark toolbar items chevron popup is correctly hooked up
+ * to the controller of a view.
+ */
+function checkBookmarksItemsChevronContextMenu() {
+  return (async function () {
+    let chevronPopup = document.getElementById("PlacesChevronPopup");
+    let shownPromise = popupShown(chevronPopup);
+    let chevron = document.getElementById("PlacesChevron");
+    EventUtils.synthesizeMouseAtCenter(chevron, {});
+    info("Waiting for bookmark toolbar item chevron popup to show");
+    await shownPromise;
+    await TestUtils.waitForCondition(() => {
+      for (let child of chevronPopup.children) {
+        if (child.style.visibility != "hidden") {
+          return true;
+        }
+      }
+      return false;
+    });
+    await checkPlacesContextMenu(chevronPopup);
+    info("Waiting for bookmark toolbar item chevron popup to close");
+    await closePopup(chevronPopup);
+  })();
+}
+
+/**
+ * Forces the window to a width that causes the nav-bar to overflow
+ * its contents. Returns a Promise that resolves as soon as the
+ * overflowable nav-bar is showing its chevron.
+ */
+function overflowEverything() {
+  info("Waiting for overflow");
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  return TestUtils.waitForCondition(() => gNavBar.hasAttribute("overflowing"));
+}
+
+/**
+ * Returns the window to its original size from the start of the test,
+ * and returns a Promise that resolves when the nav-bar is no longer
+ * overflowing.
+ */
+function stopOverflowing() {
+  info("Waiting until we stop overflowing");
+  window.resizeTo(kOriginalWindowWidth, window.outerHeight);
+  return TestUtils.waitForCondition(() => !gNavBar.hasAttribute("overflowing"));
+}
+
+/**
+ * Checks that an item with ID aID is overflowing in the nav-bar.
+ *
+ * @param aID the ID of the node to check for overflowingness.
+ */
+function checkOverflowing(aID) {
+  ok(
+    !gNavBar.querySelector("#" + aID),
+    "Item with ID " + aID + " should no longer be in the gNavBar"
+  );
+  let item = gOverflowList.querySelector("#" + aID);
+  ok(item, "Item with ID " + aID + " should be overflowing");
+  is(
+    item.getAttribute("overflowedItem"),
+    "true",
+    "Item with ID " + aID + " should have overflowedItem attribute"
+  );
+}
+
+/**
+ * Checks that an item with ID aID is not overflowing in the nav-bar.
+ *
+ * @param aID the ID of hte node to check for non-overflowingness.
+ */
+function checkNotOverflowing(aID) {
+  ok(
+    !gOverflowList.querySelector("#" + aID),
+    "Item with ID " + aID + " should no longer be overflowing"
+  );
+  let item = gNavBar.querySelector("#" + aID);
+  ok(item, "Item with ID " + aID + " should be in the nav bar");
+  ok(
+    !item.hasAttribute("overflowedItem"),
+    "Item with ID " + aID + " should not have overflowedItem attribute"
+  );
+}
+
+/**
+ * Test that overflowing the bookmarks menu button doesn't break the
+ * context menus for the Unsorted and Bookmarks Toolbar menu items.
+ */
+add_task(async function testOverflowingBookmarksButtonContextMenu() {
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+  // The DevEdition has the DevTools button in the toolbar by default. Remove it
+  // to prevent branch-specific available toolbar space.
+  CustomizableUI.removeWidgetFromArea("developer-button");
+  CustomizableUI.removeWidgetFromArea(
+    "library-button",
+    CustomizableUI.AREA_NAVBAR
+  );
+  CustomizableUI.addWidgetToArea(kBookmarksButton, CustomizableUI.AREA_NAVBAR);
+  ok(
+    !gNavBar.hasAttribute("overflowing"),
+    "Should start with a non-overflowing toolbar."
+  );
+
+  // Open the Unsorted and Bookmarks Toolbar context menus and ensure
+  // that they have views attached.
+  await checkSpecialContextMenus();
+
+  await overflowEverything();
+  checkOverflowing(kBookmarksButton);
+
+  await stopOverflowing();
+  checkNotOverflowing(kBookmarksButton);
+
+  await checkSpecialContextMenus();
+});
+
+/**
+ * Test that the bookmarks toolbar items context menu still works if moved
+ * to the menu from the overflow panel, and then back to the toolbar.
+ */
+add_task(async function testOverflowingBookmarksItemsContextMenu() {
+  info("Ensuring panel is ready.");
+  await PanelUI.ensureReady();
+
+  let bookmarksToolbarItems = document.getElementById(kBookmarksItems);
+  await gCustomizeMode.addToToolbar(bookmarksToolbarItems);
+  await TestUtils.waitForCondition(
+    () => document.getElementById("PlacesToolbar")._placesView
+  );
+  await checkPlacesContextMenu(bookmarksToolbarItems);
+
+  await overflowEverything();
+  checkOverflowing(kBookmarksItems);
+
+  await gCustomizeMode.addToPanel(bookmarksToolbarItems);
+
+  await stopOverflowing();
+
+  await gCustomizeMode.addToToolbar(bookmarksToolbarItems);
+  await TestUtils.waitForCondition(
+    () => document.getElementById("PlacesToolbar")._placesView
+  );
+  await checkPlacesContextMenu(bookmarksToolbarItems);
+});
+
+/**
+ * Test that overflowing the bookmarks toolbar items doesn't cause the
+ * context menu in the bookmarks toolbar items chevron to stop working.
+ */
+add_task(async function testOverflowingBookmarksItemsChevronContextMenu() {
+  // If it's not already there, let's move the bookmarks toolbar items to
+  // the nav-bar.
+  let bookmarksToolbarItems = document.getElementById(kBookmarksItems);
+  await gCustomizeMode.addToToolbar(bookmarksToolbarItems);
+
+  // We make the PlacesToolbarItems element be super tiny in order to force
+  // the bookmarks toolbar items into overflowing and making the chevron
+  // show itself.
+  let placesToolbarItems = document.getElementById("PlacesToolbarItems");
+  let placesChevron = document.getElementById("PlacesChevron");
+  placesToolbarItems.style.maxWidth = "10px";
+  info("Waiting for chevron to no longer be collapsed");
+  await TestUtils.waitForCondition(() => !placesChevron.collapsed);
+
+  await checkBookmarksItemsChevronContextMenu();
+
+  await overflowEverything();
+  checkOverflowing(kBookmarksItems);
+
+  await stopOverflowing();
+  checkNotOverflowing(kBookmarksItems);
+
+  await checkBookmarksItemsChevronContextMenu();
+
+  placesToolbarItems.style.removeProperty("max-width");
+});
+
+add_task(async function asyncCleanup() {
+  window.resizeTo(kOriginalWindowWidth, window.outerHeight);
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_985815_propagate_setToolbarVisibility.js b/browser/components/customizableui/test/browser_985815_propagate_setToolbarVisibility.js
new file mode 100644
index 0000000000..3b2bd13731
--- /dev/null
+++ b/browser/components/customizableui/test/browser_985815_propagate_setToolbarVisibility.js
@@ -0,0 +1,56 @@
+/* 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 () {
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+  this.otherWin = await openAndLoadWindow({ private: true }, true);
+  await startCustomizing(this.otherWin);
+  let resetButton = this.otherWin.document.getElementById(
+    "customization-reset-button"
+  );
+  ok(resetButton.disabled, "Reset button should be disabled");
+
+  if (typeof CustomizableUI.setToolbarVisibility == "function") {
+    CustomizableUI.setToolbarVisibility("PersonalToolbar", true);
+  } else {
+    setToolbarVisibility(document.getElementById("PersonalToolbar"), true);
+  }
+
+  let otherPersonalToolbar =
+    this.otherWin.document.getElementById("PersonalToolbar");
+  let personalToolbar = document.getElementById("PersonalToolbar");
+  ok(
+    !otherPersonalToolbar.collapsed,
+    "Toolbar should be uncollapsed in private window"
+  );
+  ok(
+    !personalToolbar.collapsed,
+    "Toolbar should be uncollapsed in normal window"
+  );
+  ok(!resetButton.disabled, "Reset button should be enabled");
+
+  await this.otherWin.gCustomizeMode.reset();
+
+  ok(
+    otherPersonalToolbar.collapsed,
+    "Toolbar should be collapsed in private window"
+  );
+  ok(personalToolbar.collapsed, "Toolbar should be collapsed in normal window");
+  ok(resetButton.disabled, "Reset button should be disabled");
+
+  await endCustomizing(this.otherWin);
+
+  await promiseWindowClosed(this.otherWin);
+});
+
+add_task(async function asyncCleanup() {
+  if (this.otherWin && !this.otherWin.closed) {
+    await promiseWindowClosed(this.otherWin);
+  }
+  if (!CustomizableUI.inDefaultState) {
+    CustomizableUI.reset();
+  }
+});
diff --git a/browser/components/customizableui/test/browser_987177_destroyWidget_xul.js b/browser/components/customizableui/test/browser_987177_destroyWidget_xul.js
new file mode 100644
index 0000000000..5881011b85
--- /dev/null
+++ b/browser/components/customizableui/test/browser_987177_destroyWidget_xul.js
@@ -0,0 +1,35 @@
+/* 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";
+
+const BUTTONID = "test-XUL-wrapper-destroyWidget";
+
+add_task(function () {
+  let btn = createDummyXULButton(BUTTONID, "XUL btn");
+  gNavToolbox.palette.appendChild(btn);
+  let firstWrapper = CustomizableUI.getWidget(BUTTONID).forWindow(window);
+  ok(firstWrapper, "Should get a wrapper");
+  ok(firstWrapper.node, "Node should be there on first wrapper.");
+
+  btn.remove();
+  CustomizableUI.destroyWidget(BUTTONID);
+  let secondWrapper = CustomizableUI.getWidget(BUTTONID).forWindow(window);
+  isnot(
+    firstWrapper,
+    secondWrapper,
+    "Wrappers should be different after destroyWidget call."
+  );
+  ok(!firstWrapper.node, "No node should be there on old wrapper.");
+  ok(!secondWrapper.node, "No node should be there on new wrapper.");
+
+  btn = createDummyXULButton(BUTTONID, "XUL btn");
+  gNavToolbox.palette.appendChild(btn);
+  let thirdWrapper = CustomizableUI.getWidget(BUTTONID).forWindow(window);
+  ok(thirdWrapper, "Should get a wrapper");
+  is(secondWrapper, thirdWrapper, "Should get the second wrapper again.");
+  ok(firstWrapper.node, "Node should be there on old wrapper.");
+  ok(secondWrapper.node, "Node should be there on second wrapper.");
+  ok(thirdWrapper.node, "Node should be there on third wrapper.");
+});
diff --git a/browser/components/customizableui/test/browser_987177_xul_wrapper_updating.js b/browser/components/customizableui/test/browser_987177_xul_wrapper_updating.js
new file mode 100644
index 0000000000..9ef22c4e1b
--- /dev/null
+++ b/browser/components/customizableui/test/browser_987177_xul_wrapper_updating.js
@@ -0,0 +1,142 @@
+/* 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";
+
+const BUTTONID = "test-XUL-wrapper-widget";
+add_task(function () {
+  let btn = createDummyXULButton(BUTTONID, "XUL btn");
+  gNavToolbox.palette.appendChild(btn);
+  let groupWrapper = CustomizableUI.getWidget(BUTTONID);
+  ok(groupWrapper, "Should get a group wrapper");
+  let singleWrapper = groupWrapper.forWindow(window);
+  ok(singleWrapper, "Should get a single wrapper");
+  is(singleWrapper.node, btn, "Node should be in the wrapper");
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, btn, "Button should be that instance.");
+
+  CustomizableUI.addWidgetToArea(BUTTONID, CustomizableUI.AREA_NAVBAR);
+
+  let otherSingleWrapper = groupWrapper.forWindow(window);
+  is(
+    singleWrapper,
+    otherSingleWrapper,
+    "Should get the same wrapper after adding the node to the navbar."
+  );
+  is(singleWrapper.node, btn, "Node should be in the wrapper");
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, btn, "Button should be that instance.");
+
+  CustomizableUI.removeWidgetFromArea(BUTTONID);
+
+  otherSingleWrapper = groupWrapper.forWindow(window);
+  isnot(
+    singleWrapper,
+    otherSingleWrapper,
+    "Shouldn't get the same wrapper after removing it from the navbar."
+  );
+  singleWrapper = otherSingleWrapper;
+  is(singleWrapper.node, btn, "Node should be in the wrapper");
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, btn, "Button should be that instance.");
+
+  btn.remove();
+  otherSingleWrapper = groupWrapper.forWindow(window);
+  is(
+    singleWrapper,
+    otherSingleWrapper,
+    "Should get the same wrapper after physically removing the node."
+  );
+  is(
+    singleWrapper.node,
+    null,
+    "Wrapper's node should be null now that it's left the DOM."
+  );
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, null, "That instance should be null.");
+
+  btn = createDummyXULButton(BUTTONID, "XUL btn");
+  gNavToolbox.palette.appendChild(btn);
+  otherSingleWrapper = groupWrapper.forWindow(window);
+  is(
+    singleWrapper,
+    otherSingleWrapper,
+    "Should get the same wrapper after readding the node."
+  );
+  is(singleWrapper.node, btn, "Node should be in the wrapper");
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, btn, "Button should be that instance.");
+
+  CustomizableUI.addWidgetToArea(BUTTONID, CustomizableUI.AREA_NAVBAR);
+
+  otherSingleWrapper = groupWrapper.forWindow(window);
+  is(
+    singleWrapper,
+    otherSingleWrapper,
+    "Should get the same wrapper after adding the node to the navbar."
+  );
+  is(singleWrapper.node, btn, "Node should be in the wrapper");
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, btn, "Button should be that instance.");
+
+  CustomizableUI.removeWidgetFromArea(BUTTONID);
+
+  otherSingleWrapper = groupWrapper.forWindow(window);
+  isnot(
+    singleWrapper,
+    otherSingleWrapper,
+    "Shouldn't get the same wrapper after removing it from the navbar."
+  );
+  singleWrapper = otherSingleWrapper;
+  is(singleWrapper.node, btn, "Node should be in the wrapper");
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, btn, "Button should be that instance.");
+
+  btn.remove();
+  otherSingleWrapper = groupWrapper.forWindow(window);
+  is(
+    singleWrapper,
+    otherSingleWrapper,
+    "Should get the same wrapper after physically removing the node."
+  );
+  is(
+    singleWrapper.node,
+    null,
+    "Wrapper's node should be null now that it's left the DOM."
+  );
+  is(
+    groupWrapper.instances.length,
+    1,
+    "There should be 1 instance on the group wrapper"
+  );
+  is(groupWrapper.instances[0].node, null, "That instance should be null.");
+});
diff --git a/browser/components/customizableui/test/browser_987492_window_api.js b/browser/components/customizableui/test/browser_987492_window_api.js
new file mode 100644
index 0000000000..5e69573d60
--- /dev/null
+++ b/browser/components/customizableui/test/browser_987492_window_api.js
@@ -0,0 +1,83 @@
+/* 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 testOneWindow() {
+  let windows = [];
+  for (let win of CustomizableUI.windows) {
+    windows.push(win);
+  }
+  is(windows.length, 1, "Should have one customizable window");
+});
+
+add_task(async function testOpenCloseWindow() {
+  let newWindow = null;
+  let openListener = {
+    onWindowOpened(window) {
+      newWindow = window;
+    },
+  };
+  CustomizableUI.addListener(openListener);
+
+  {
+    let win = await openAndLoadWindow(null, true);
+    is(
+      newWindow,
+      win,
+      "onWindowOpen event should have received expected window"
+    );
+    isnot(newWindow, null, "Should have gotten onWindowOpen event");
+  }
+
+  CustomizableUI.removeListener(openListener);
+
+  let windows = [];
+  for (let win of CustomizableUI.windows) {
+    windows.push(win);
+  }
+  is(windows.length, 2, "Should have two customizable windows");
+  isnot(
+    windows.indexOf(window),
+    -1,
+    "Current window should be in window collection."
+  );
+  isnot(
+    windows.indexOf(newWindow),
+    -1,
+    "New window should be in window collection."
+  );
+
+  let closedWindow = null;
+  let closeListener = {
+    onWindowClosed(window) {
+      closedWindow = window;
+    },
+  };
+  CustomizableUI.addListener(closeListener);
+  await promiseWindowClosed(newWindow);
+  isnot(closedWindow, null, "Should have gotten onWindowClosed event");
+  is(
+    newWindow,
+    closedWindow,
+    "Closed window should match previously opened window"
+  );
+  CustomizableUI.removeListener(closeListener);
+
+  windows = [];
+  for (let win of CustomizableUI.windows) {
+    windows.push(win);
+  }
+  is(windows.length, 1, "Should have one customizable window");
+  isnot(
+    windows.indexOf(window),
+    -1,
+    "Current window should be in window collection."
+  );
+  is(
+    windows.indexOf(closedWindow),
+    -1,
+    "Closed window should not be in window collection."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_987640_charEncoding.js b/browser/components/customizableui/test/browser_987640_charEncoding.js
new file mode 100644
index 0000000000..65e38a0b85
--- /dev/null
+++ b/browser/components/customizableui/test/browser_987640_charEncoding.js
@@ -0,0 +1,78 @@
+/* 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";
+
+const TEST_PAGE =
+  "http://mochi.test:8888/browser/browser/components/customizableui/test/support/test_967000_charEncoding_page.html";
+
+add_task(async function () {
+  info("Check Character Encoding panel functionality");
+
+  // add the Character Encoding button to the panel
+  CustomizableUI.addWidgetToArea(
+    "characterencoding-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  let newTab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_PAGE,
+    true,
+    true
+  );
+
+  await document.getElementById("nav-bar").overflowable.show();
+  let charEncodingButton = document.getElementById("characterencoding-button");
+
+  ok(
+    !charEncodingButton.hasAttribute("disabled"),
+    "The encoding button should be enabled"
+  );
+
+  let browserStopPromise = BrowserTestUtils.browserStopped(gBrowser, TEST_PAGE);
+  charEncodingButton.click();
+  await browserStopPromise;
+  is(
+    gBrowser.selectedBrowser.characterSet,
+    "UTF-8",
+    "The encoding should be changed to UTF-8"
+  );
+  ok(
+    !gBrowser.selectedBrowser.mayEnableCharacterEncodingMenu,
+    "The encoding menu should be disabled"
+  );
+
+  is(
+    charEncodingButton.getAttribute("disabled"),
+    "true",
+    "We should disable the encoding button in toolbar"
+  );
+
+  CustomizableUI.removeWidgetFromArea("characterencoding-button");
+  CustomizableUI.addWidgetToArea(
+    "characterencoding-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  await waitForOverflowButtonShown();
+  await document.getElementById("nav-bar").overflowable.show();
+  charEncodingButton = document.getElementById("characterencoding-button");
+
+  // check the encoding menu again
+  is(
+    charEncodingButton.getAttribute("disabled"),
+    "true",
+    "We should disable the encoding button in overflow menu"
+  );
+
+  BrowserTestUtils.removeTab(newTab);
+});
+
+add_task(async function asyncCleanup() {
+  // reset the panel to the default state
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "The UI is in default state again.");
+});
diff --git a/browser/components/customizableui/test/browser_989338_saved_placements_not_resaved.js b/browser/components/customizableui/test/browser_989338_saved_placements_not_resaved.js
new file mode 100644
index 0000000000..d2b87a7a31
--- /dev/null
+++ b/browser/components/customizableui/test/browser_989338_saved_placements_not_resaved.js
@@ -0,0 +1,70 @@
+/* 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";
+
+const BUTTONID = "test-widget-saved-earlier";
+const AREAID = "test-area-saved-earlier";
+
+var hadSavedState;
+function test() {
+  let gSavedState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+  hadSavedState = gSavedState != null;
+  if (!hadSavedState) {
+    gSavedState = { placements: {} };
+    CustomizableUI.setTestOnlyInternalProp("gSavedState", gSavedState);
+  }
+  gSavedState.placements[AREAID] = [BUTTONID];
+  // Put bogus stuff in the saved state for the nav-bar, so as to check the current placements
+  // override this one...
+  gSavedState.placements[CustomizableUI.AREA_NAVBAR] = ["bogus-navbar-item"];
+
+  CustomizableUI.setTestOnlyInternalProp("gDirty", true);
+  CustomizableUI.getTestOnlyInternalProp("CustomizableUIInternal").saveState();
+
+  let newSavedState = JSON.parse(
+    Services.prefs.getCharPref("browser.uiCustomization.state")
+  );
+  let savedArea = Array.isArray(newSavedState.placements[AREAID]);
+  ok(
+    savedArea,
+    "Should have re-saved the state, even though the area isn't registered"
+  );
+
+  if (savedArea) {
+    placementArraysEqual(AREAID, newSavedState.placements[AREAID], [BUTTONID]);
+  }
+  ok(
+    !CustomizableUI.getTestOnlyInternalProp("gPlacements").has(AREAID),
+    "Placements map shouldn't have been affected"
+  );
+
+  let savedNavbar = Array.isArray(
+    newSavedState.placements[CustomizableUI.AREA_NAVBAR]
+  );
+  ok(savedNavbar, "Should have saved nav-bar contents");
+  if (savedNavbar) {
+    placementArraysEqual(
+      CustomizableUI.AREA_NAVBAR,
+      newSavedState.placements[CustomizableUI.AREA_NAVBAR],
+      CustomizableUI.getWidgetIdsInArea(CustomizableUI.AREA_NAVBAR)
+    );
+  }
+}
+
+registerCleanupFunction(function () {
+  if (!hadSavedState) {
+    CustomizableUI.setTestOnlyInternalProp("gSavedState", null);
+  } else {
+    let gSavedState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+    let savedPlacements = gSavedState.placements;
+    delete savedPlacements[AREAID];
+    let realNavBarPlacements = CustomizableUI.getWidgetIdsInArea(
+      CustomizableUI.AREA_NAVBAR
+    );
+    savedPlacements[CustomizableUI.AREA_NAVBAR] = realNavBarPlacements;
+  }
+  CustomizableUI.setTestOnlyInternalProp("gDirty", true);
+  CustomizableUI.getTestOnlyInternalProp("CustomizableUIInternal").saveState();
+});
diff --git a/browser/components/customizableui/test/browser_989751_subviewbutton_class.js b/browser/components/customizableui/test/browser_989751_subviewbutton_class.js
new file mode 100644
index 0000000000..d97017cfcd
--- /dev/null
+++ b/browser/components/customizableui/test/browser_989751_subviewbutton_class.js
@@ -0,0 +1,91 @@
+/* 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";
+
+const kCustomClass = "acustomclassnoonewilluse";
+const kDevPanelId = "PanelUI-developer-tools";
+var tempElement = null;
+
+function insertClassNameToMenuChildren(parentMenu) {
+  // Skip hidden menuitem elements, not copied via fillSubviewFromMenuItems.
+  let el = parentMenu.querySelector("menuitem:not([hidden])");
+  el.classList.add(kCustomClass);
+  tempElement = el;
+}
+
+function checkSubviewButtonClass(menuId, buttonId, subviewId) {
+  return async function () {
+    // Initialize DevTools before starting the test in order to create menuitems in
+    // menuWebDeveloperPopup.
+    ChromeUtils.importESModule(
+      "resource://devtools/shared/loader/Loader.sys.mjs"
+    ).require("devtools/client/framework/devtools-browser");
+
+    info(
+      "Checking for items without the subviewbutton class in " +
+        buttonId +
+        " widget"
+    );
+    let menu = document.getElementById(menuId);
+    insertClassNameToMenuChildren(menu);
+
+    CustomizableUI.addWidgetToArea(
+      buttonId,
+      CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+    );
+
+    await waitForOverflowButtonShown();
+
+    await document.getElementById("nav-bar").overflowable.show();
+
+    let button = document.getElementById(buttonId);
+    button.click();
+
+    await BrowserTestUtils.waitForEvent(PanelUI.overflowPanel, "ViewShown");
+    let subview = document.getElementById(subviewId);
+    ok(subview.firstElementChild, "Subview should have a kid");
+
+    // The Developer Panel contains the Customize Toolbar item,
+    // as well as the Developer Tools items (bug 1703150). We only want to query for
+    // the Developer Tools items in this case.
+    let query = "#appmenu-developer-tools-view toolbarbutton";
+    let subviewchildren = subview.querySelectorAll(query);
+
+    for (let i = 0; i < subviewchildren.length; i++) {
+      let item = subviewchildren[i];
+      let itemReadable =
+        "Item '" + item.label + "' (classes: " + item.className + ")";
+      ok(
+        item.classList.contains("subviewbutton"),
+        itemReadable + " should have the subviewbutton class."
+      );
+      if (i == 0) {
+        ok(
+          item.classList.contains(kCustomClass),
+          itemReadable + " should still have its own class, too."
+        );
+      }
+    }
+
+    let panelHiddenPromise = promiseOverflowHidden(window);
+    PanelUI.overflowPanel.hidePopup();
+    await panelHiddenPromise;
+
+    CustomizableUI.reset();
+  };
+}
+
+add_task(
+  checkSubviewButtonClass(
+    "menuWebDeveloperPopup",
+    "developer-button",
+    kDevPanelId
+  )
+);
+
+registerCleanupFunction(function () {
+  tempElement.classList.remove(kCustomClass);
+  tempElement = null;
+});
diff --git a/browser/components/customizableui/test/browser_992747_toggle_noncustomizable_toolbar.js b/browser/components/customizableui/test/browser_992747_toggle_noncustomizable_toolbar.js
new file mode 100644
index 0000000000..d89845f03b
--- /dev/null
+++ b/browser/components/customizableui/test/browser_992747_toggle_noncustomizable_toolbar.js
@@ -0,0 +1,25 @@
+/* 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";
+
+const TOOLBARID = "test-noncustomizable-toolbar-for-toggling";
+function test() {
+  let tb = document.createXULElement("toolbar");
+  tb.id = TOOLBARID;
+  gNavToolbox.appendChild(tb);
+  try {
+    CustomizableUI.setToolbarVisibility(TOOLBARID, false);
+  } catch (ex) {
+    ok(false, "Should not throw exceptions trying to set toolbar visibility.");
+  }
+  is(tb.getAttribute("collapsed"), "true", "Toolbar should be collapsed");
+  try {
+    CustomizableUI.setToolbarVisibility(TOOLBARID, true);
+  } catch (ex) {
+    ok(false, "Should not throw exceptions trying to set toolbar visibility.");
+  }
+  is(tb.getAttribute("collapsed"), "false", "Toolbar should be uncollapsed");
+  tb.remove();
+}
diff --git a/browser/components/customizableui/test/browser_993322_widget_notoolbar.js b/browser/components/customizableui/test/browser_993322_widget_notoolbar.js
new file mode 100644
index 0000000000..5e6cc65585
--- /dev/null
+++ b/browser/components/customizableui/test/browser_993322_widget_notoolbar.js
@@ -0,0 +1,59 @@
+/* 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";
+
+const BUTTONID = "test-API-created-widget-toolbar-gone";
+const TOOLBARID = "test-API-created-extra-toolbar";
+
+add_task(async function () {
+  let toolbar = createToolbarWithPlacements(TOOLBARID, []);
+  CustomizableUI.addWidgetToArea(BUTTONID, TOOLBARID);
+  is(
+    CustomizableUI.getPlacementOfWidget(BUTTONID).area,
+    TOOLBARID,
+    "Should be on toolbar"
+  );
+  is(toolbar.children.length, 0, "Toolbar has no kid");
+
+  CustomizableUI.unregisterArea(TOOLBARID);
+  CustomizableUI.createWidget({
+    id: BUTTONID,
+    label: "Test widget toolbar gone",
+  });
+
+  let currentWidget = CustomizableUI.getWidget(BUTTONID);
+
+  await startCustomizing();
+  let buttonNode = document.getElementById(BUTTONID);
+  ok(buttonNode, "Should find button in window");
+  if (buttonNode) {
+    is(
+      buttonNode.parentNode.localName,
+      "toolbarpaletteitem",
+      "Node should be wrapped"
+    );
+    is(
+      buttonNode.parentNode.getAttribute("place"),
+      "palette",
+      "Node should be in palette"
+    );
+    is(
+      buttonNode,
+      gNavToolbox.palette.querySelector("#" + BUTTONID),
+      "Node should really be in palette."
+    );
+  }
+  is(
+    currentWidget.forWindow(window).node,
+    buttonNode,
+    "Should have the same node for customize mode"
+  );
+  await endCustomizing();
+
+  CustomizableUI.destroyWidget(BUTTONID);
+  CustomizableUI.unregisterArea(TOOLBARID, true);
+  toolbar.remove();
+  gAddedToolbars.clear();
+});
diff --git a/browser/components/customizableui/test/browser_995164_registerArea_during_customize_mode.js b/browser/components/customizableui/test/browser_995164_registerArea_during_customize_mode.js
new file mode 100644
index 0000000000..8829611083
--- /dev/null
+++ b/browser/components/customizableui/test/browser_995164_registerArea_during_customize_mode.js
@@ -0,0 +1,278 @@
+/* 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";
+
+const TOOLBARID = "test-toolbar-added-during-customize-mode";
+
+// The ID of a button that is not placed (ie, is in the palette) by default
+const kNonPlacedWidgetId = "open-file-button";
+
+add_task(async function () {
+  await startCustomizing();
+  let toolbar = createToolbarWithPlacements(TOOLBARID, []);
+  CustomizableUI.addWidgetToArea(kNonPlacedWidgetId, TOOLBARID);
+  let button = document.getElementById(kNonPlacedWidgetId);
+  ok(button, "Button should exist.");
+  is(
+    button.parentNode.localName,
+    "toolbarpaletteitem",
+    "Button's parent node should be a wrapper."
+  );
+
+  simulateItemDrag(button, gNavToolbox.palette);
+  ok(
+    !CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId),
+    "Button moved to the palette"
+  );
+  ok(
+    gNavToolbox.palette.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button really is in palette."
+  );
+
+  button.scrollIntoView();
+  simulateItemDrag(button, toolbar);
+  ok(
+    CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId),
+    "Button moved out of palette"
+  );
+  is(
+    CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId).area,
+    TOOLBARID,
+    "Button's back on toolbar"
+  );
+  ok(
+    toolbar.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button really is on toolbar."
+  );
+
+  await endCustomizing();
+  isnot(
+    button.parentNode.localName,
+    "toolbarpaletteitem",
+    "Button's parent node should not be a wrapper outside customize mode."
+  );
+  await startCustomizing();
+
+  is(
+    button.parentNode.localName,
+    "toolbarpaletteitem",
+    "Button's parent node should be a wrapper back in customize mode."
+  );
+
+  simulateItemDrag(button, gNavToolbox.palette);
+  ok(
+    !CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId),
+    "Button moved to the palette"
+  );
+  ok(
+    gNavToolbox.palette.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button really is in palette."
+  );
+
+  ok(
+    !CustomizableUI.inDefaultState,
+    "Not in default state while toolbar is not collapsed yet."
+  );
+  setToolbarVisibility(toolbar, false);
+  ok(
+    CustomizableUI.inDefaultState,
+    "In default state while toolbar is collapsed."
+  );
+
+  setToolbarVisibility(toolbar, true);
+
+  info(
+    "Check that removing the area registration from within customize mode works"
+  );
+  CustomizableUI.unregisterArea(TOOLBARID);
+  ok(
+    CustomizableUI.inDefaultState,
+    "Now that the toolbar is no longer registered, should be in default state."
+  );
+  ok(
+    !gCustomizeMode.areas.has(toolbar),
+    "Toolbar shouldn't be known to customize mode."
+  );
+
+  CustomizableUI.registerArea(TOOLBARID, { defaultPlacements: [] });
+  CustomizableUI.registerToolbarNode(toolbar, []);
+  ok(
+    !CustomizableUI.inDefaultState,
+    "Now that the toolbar is registered again, should no longer be in default state."
+  );
+  ok(
+    gCustomizeMode.areas.has(toolbar),
+    "Toolbar should be known to customize mode again."
+  );
+
+  button.scrollIntoView();
+  simulateItemDrag(button, toolbar);
+  ok(
+    CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId),
+    "Button moved out of palette"
+  );
+  is(
+    CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId).area,
+    TOOLBARID,
+    "Button's back on toolbar"
+  );
+  ok(
+    toolbar.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button really is on toolbar."
+  );
+
+  let otherWin = await openAndLoadWindow({}, true);
+  let otherTB = otherWin.document.createXULElement("toolbar");
+  otherTB.id = TOOLBARID;
+  otherTB.setAttribute("customizable", "true");
+  let wasInformedCorrectlyOfAreaAppearing = false;
+  let listener = {
+    onAreaNodeRegistered(aArea, aNode) {
+      if (aNode == otherTB) {
+        wasInformedCorrectlyOfAreaAppearing = true;
+      }
+    },
+  };
+  CustomizableUI.addListener(listener);
+  otherWin.gNavToolbox.appendChild(otherTB);
+  CustomizableUI.registerToolbarNode(otherTB);
+  ok(
+    wasInformedCorrectlyOfAreaAppearing,
+    "Should have been told area was registered."
+  );
+  CustomizableUI.removeListener(listener);
+
+  ok(
+    otherTB.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button is on other toolbar, too."
+  );
+
+  simulateItemDrag(button, gNavToolbox.palette);
+  ok(
+    !CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId),
+    "Button moved to the palette"
+  );
+  ok(
+    gNavToolbox.palette.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button really is in palette."
+  );
+  ok(
+    !otherTB.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button is in palette in other window, too."
+  );
+
+  button.scrollIntoView();
+  simulateItemDrag(button, toolbar);
+  ok(
+    CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId),
+    "Button moved out of palette"
+  );
+  is(
+    CustomizableUI.getPlacementOfWidget(kNonPlacedWidgetId).area,
+    TOOLBARID,
+    "Button's back on toolbar"
+  );
+  ok(
+    toolbar.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button really is on toolbar."
+  );
+  ok(
+    otherTB.querySelector(`#${kNonPlacedWidgetId}`),
+    "Button is on other toolbar, too."
+  );
+
+  let wasInformedCorrectlyOfAreaDisappearing = false;
+  // XXXgijs So we could be using promiseWindowClosed here. However, after
+  // repeated random oranges, I'm instead relying on onWindowClosed below to
+  // fire appropriately - it is linked to an unload event as well, and so
+  // reusing it prevents a potential race between unload handlers where the
+  // one from promiseWindowClosed could fire before the onWindowClosed
+  // (and therefore onAreaNodeRegistered) one, causing the test to fail.
+  let windowClosed = await new Promise(resolve => {
+    listener = {
+      onAreaNodeUnregistered(aArea, aNode, aReason) {
+        if (aArea == TOOLBARID) {
+          is(aNode, otherTB, "Should be informed about other toolbar");
+          is(
+            aReason,
+            CustomizableUI.REASON_WINDOW_CLOSED,
+            "Reason should be correct."
+          );
+          wasInformedCorrectlyOfAreaDisappearing =
+            aReason === CustomizableUI.REASON_WINDOW_CLOSED;
+        }
+      },
+      onWindowClosed(aWindow) {
+        if (aWindow == otherWin) {
+          resolve(aWindow);
+        } else {
+          info("Other window was closed!");
+          info(
+            "Other window title: " +
+              (aWindow.document && aWindow.document.title)
+          );
+          info(
+            "Our window title: " +
+              (otherWin.document && otherWin.document.title)
+          );
+        }
+      },
+    };
+    CustomizableUI.addListener(listener);
+    otherWin.close();
+  });
+
+  is(
+    windowClosed,
+    otherWin,
+    "Window should have sent onWindowClosed notification."
+  );
+  ok(
+    wasInformedCorrectlyOfAreaDisappearing,
+    "Should be told about window closing."
+  );
+  // Closing the other window should not be counted against this window's customize mode:
+  is(
+    button.parentNode.localName,
+    "toolbarpaletteitem",
+    "Button's parent node should still be a wrapper."
+  );
+  ok(
+    gCustomizeMode.areas.has(toolbar),
+    "Toolbar should still be a customizable area for this customize mode instance."
+  );
+
+  await gCustomizeMode.reset();
+
+  await endCustomizing();
+
+  CustomizableUI.removeListener(listener);
+  wasInformedCorrectlyOfAreaDisappearing = false;
+  listener = {
+    onAreaNodeUnregistered(aArea, aNode, aReason) {
+      if (aArea == TOOLBARID) {
+        is(aNode, toolbar, "Should be informed about this window's toolbar");
+        is(
+          aReason,
+          CustomizableUI.REASON_AREA_UNREGISTERED,
+          "Reason for final removal should be correct."
+        );
+        wasInformedCorrectlyOfAreaDisappearing =
+          aReason === CustomizableUI.REASON_AREA_UNREGISTERED;
+      }
+    },
+  };
+  CustomizableUI.addListener(listener);
+  removeCustomToolbars();
+  ok(
+    wasInformedCorrectlyOfAreaDisappearing,
+    "Should be told about area being unregistered."
+  );
+  CustomizableUI.removeListener(listener);
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be fine after exiting customize mode."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_996364_registerArea_different_properties.js b/browser/components/customizableui/test/browser_996364_registerArea_different_properties.js
new file mode 100644
index 0000000000..7d8ea59150
--- /dev/null
+++ b/browser/components/customizableui/test/browser_996364_registerArea_different_properties.js
@@ -0,0 +1,142 @@
+/* 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";
+
+// Calling CustomizableUI.registerArea twice with no
+// properties should not throw an exception.
+add_task(function () {
+  try {
+    CustomizableUI.registerArea("area-996364", {});
+    CustomizableUI.registerArea("area-996364", {});
+  } catch (ex) {
+    ok(false, ex.message);
+  }
+
+  CustomizableUI.unregisterArea("area-996364", true);
+});
+
+add_task(function () {
+  let exceptionThrown = false;
+  try {
+    CustomizableUI.registerArea("area-996364-2", {
+      type: CustomizableUI.TYPE_TOOLBAR,
+      defaultCollapsed: "false",
+    });
+  } catch (ex) {
+    exceptionThrown = true;
+  }
+  ok(
+    exceptionThrown,
+    "defaultCollapsed is not allowed as an external property"
+  );
+
+  // No need to unregister the area because registration fails.
+});
+
+add_task(function () {
+  let exceptionThrown;
+  try {
+    CustomizableUI.registerArea("area-996364-3", {
+      type: CustomizableUI.TYPE_TOOLBAR,
+    });
+    CustomizableUI.registerArea("area-996364-3", {
+      type: CustomizableUI.TYPE_PANEL,
+    });
+  } catch (ex) {
+    exceptionThrown = ex;
+  }
+  ok(
+    exceptionThrown,
+    "Exception expected, an area cannot change types: " +
+      (exceptionThrown ? exceptionThrown : "[no exception]")
+  );
+
+  CustomizableUI.unregisterArea("area-996364-3", true);
+});
+
+add_task(function () {
+  let exceptionThrown;
+  try {
+    CustomizableUI.registerArea("area-996364-4", {
+      type: CustomizableUI.TYPE_PANEL,
+    });
+    CustomizableUI.registerArea("area-996364-4", {
+      type: CustomizableUI.TYPE_TOOLBAR,
+    });
+  } catch (ex) {
+    exceptionThrown = ex;
+  }
+  ok(
+    exceptionThrown,
+    "Exception expected, an area cannot change types: " +
+      (exceptionThrown ? exceptionThrown : "[no exception]")
+  );
+
+  CustomizableUI.unregisterArea("area-996364-4", true);
+});
+
+add_task(function () {
+  let exceptionThrown;
+  try {
+    CustomizableUI.registerArea("area-996899-1", {
+      anchor: "PanelUI-menu-button",
+      type: CustomizableUI.TYPE_PANEL,
+      defaultPlacements: [],
+    });
+    CustomizableUI.registerArea("area-996899-1", {
+      anchor: "home-button",
+      type: CustomizableUI.TYPE_PANEL,
+      defaultPlacements: [],
+    });
+  } catch (ex) {
+    exceptionThrown = ex;
+  }
+  ok(
+    !exceptionThrown,
+    "Changing anchors shouldn't throw an exception: " +
+      (exceptionThrown ? exceptionThrown : "[no exception]")
+  );
+  CustomizableUI.unregisterArea("area-996899-1", true);
+});
+
+add_task(function () {
+  let exceptionThrown;
+  try {
+    CustomizableUI.registerArea("area-996899-2", {
+      anchor: "PanelUI-menu-button",
+      type: CustomizableUI.TYPE_PANEL,
+      defaultPlacements: [],
+    });
+    CustomizableUI.registerArea("area-996899-2", {
+      anchor: "PanelUI-menu-button",
+      type: CustomizableUI.TYPE_PANEL,
+      defaultPlacements: ["new-window-button"],
+    });
+  } catch (ex) {
+    exceptionThrown = ex;
+  }
+  ok(
+    !exceptionThrown,
+    "Changing defaultPlacements shouldn't throw an exception: " +
+      (exceptionThrown ? exceptionThrown : "[no exception]")
+  );
+  CustomizableUI.unregisterArea("area-996899-2", true);
+});
+
+add_task(function () {
+  let exceptionThrown;
+  try {
+    CustomizableUI.registerArea("area-996899-4", { overflowable: true });
+    CustomizableUI.registerArea("area-996899-4", { overflowable: false });
+  } catch (ex) {
+    exceptionThrown = ex;
+  }
+  ok(
+    exceptionThrown,
+    "Changing 'overflowable' should throw an exception: " +
+      (exceptionThrown ? exceptionThrown : "[no exception]")
+  );
+  CustomizableUI.unregisterArea("area-996899-4", true);
+});
diff --git a/browser/components/customizableui/test/browser_996635_remove_non_widgets.js b/browser/components/customizableui/test/browser_996635_remove_non_widgets.js
new file mode 100644
index 0000000000..80f68433e8
--- /dev/null
+++ b/browser/components/customizableui/test/browser_996635_remove_non_widgets.js
@@ -0,0 +1,54 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// NB: This is testing what happens if something that /isn't/ a customizable
+// widget gets used in CustomizableUI APIs. Don't use this as an example of
+// what should happen in a "normal" case or how you should use the API.
+function test() {
+  // First create a button that isn't customizable, and add it in the nav-bar,
+  // but not in the customizable part of it (the customization target) but
+  // next to the main (hamburger) menu button.
+  const buttonID = "Test-non-widget-non-removable-button";
+  let btn = document.createXULElement("toolbarbutton");
+  btn.id = buttonID;
+  btn.label = "Hi";
+  btn.setAttribute("style", "width: 20px; height: 20px; background-color: red");
+  document.getElementById("nav-bar").appendChild(btn);
+  registerCleanupFunction(function () {
+    btn.remove();
+  });
+
+  // Now try to add this non-customizable button to the tabstrip. This will
+  // update the internal bookkeeping (ie placements) information, but shouldn't
+  // move the node.
+  CustomizableUI.addWidgetToArea(buttonID, CustomizableUI.AREA_TABSTRIP);
+  let placement = CustomizableUI.getPlacementOfWidget(buttonID);
+  // Check our bookkeeping
+  ok(placement, "Button should be placed");
+  is(
+    placement && placement.area,
+    CustomizableUI.AREA_TABSTRIP,
+    "Should be placed on tabstrip."
+  );
+  // Check we didn't move the node.
+  is(
+    btn.parentNode && btn.parentNode.id,
+    "nav-bar",
+    "Actual button should still be on navbar."
+  );
+
+  // Now remove the node again. This should remove the bookkeeping, but again
+  // not affect the actual node.
+  CustomizableUI.removeWidgetFromArea(buttonID);
+  placement = CustomizableUI.getPlacementOfWidget(buttonID);
+  // Check our bookkeeping:
+  ok(!placement, "Button should no longer have a placement.");
+  // Check our node.
+  is(
+    btn.parentNode && btn.parentNode.id,
+    "nav-bar",
+    "Actual button should still be on navbar."
+  );
+}
diff --git a/browser/components/customizableui/test/browser_PanelMultiView.js b/browser/components/customizableui/test/browser_PanelMultiView.js
new file mode 100644
index 0000000000..80778b94b0
--- /dev/null
+++ b/browser/components/customizableui/test/browser_PanelMultiView.js
@@ -0,0 +1,566 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Unit tests for the PanelMultiView module.
+ */
+
+const PANELS_COUNT = 2;
+let gPanelAnchors = [];
+let gPanels = [];
+let gPanelMultiViews = [];
+
+const PANELVIEWS_COUNT = 4;
+let gPanelViews = [];
+let gPanelViewLabels = [];
+
+const EVENT_TYPES = [
+  "popupshown",
+  "popuphidden",
+  "PanelMultiViewHidden",
+  "ViewShowing",
+  "ViewShown",
+  "ViewHiding",
+];
+
+/**
+ * Checks that the element is displayed, including the state of the popup where
+ * the element is located. This can trigger a synchronous reflow if necessary,
+ * because even though the code under test is designed to avoid synchronous
+ * reflows, it can raise completion events while a layout flush is still needed.
+ *
+ * In production code, event handlers for ViewShown have to wait for a flush if
+ * they need to read style or layout information, like other code normally does.
+ */
+function is_visible(element) {
+  let win = element.ownerGlobal;
+  let style = win.getComputedStyle(element);
+  if (style.display == "none") {
+    return false;
+  }
+  if (style.visibility != "visible") {
+    return false;
+  }
+  if (win.XULPopupElement.isInstance(element) && element.state != "open") {
+    return false;
+  }
+
+  // Hiding a parent element will hide all its children
+  if (element.parentNode != element.ownerDocument) {
+    return is_visible(element.parentNode);
+  }
+
+  return true;
+}
+
+/**
+ * Checks whether the label in the specified view is visible.
+ */
+function assertLabelVisible(viewIndex, expectedVisible) {
+  Assert.equal(
+    is_visible(gPanelViewLabels[viewIndex]),
+    expectedVisible,
+    `Visibility of label in view ${viewIndex}`
+  );
+}
+
+/**
+ * Opens the specified view as the main view in the specified panel.
+ */
+async function openPopup(panelIndex, viewIndex) {
+  gPanelMultiViews[panelIndex].setAttribute(
+    "mainViewId",
+    gPanelViews[viewIndex].id
+  );
+
+  let promiseShown = BrowserTestUtils.waitForEvent(
+    gPanelViews[viewIndex],
+    "ViewShown"
+  );
+  PanelMultiView.openPopup(
+    gPanels[panelIndex],
+    gPanelAnchors[panelIndex],
+    "bottomright topright"
+  );
+  await promiseShown;
+
+  Assert.ok(PanelView.forNode(gPanelViews[viewIndex]).active);
+  assertLabelVisible(viewIndex, true);
+}
+
+/**
+ * Closes the specified panel.
+ */
+async function hidePopup(panelIndex) {
+  gPanelMultiViews[panelIndex].setAttribute(
+    "mainViewId",
+    gPanelViews[panelIndex].id
+  );
+
+  let promiseHidden = BrowserTestUtils.waitForEvent(
+    gPanels[panelIndex],
+    "popuphidden"
+  );
+  PanelMultiView.hidePopup(gPanels[panelIndex]);
+  await promiseHidden;
+}
+
+/**
+ * Opens the specified subview in the specified panel.
+ */
+async function showSubView(panelIndex, viewIndex) {
+  let promiseShown = BrowserTestUtils.waitForEvent(
+    gPanelViews[viewIndex],
+    "ViewShown"
+  );
+  gPanelMultiViews[panelIndex].showSubView(gPanelViews[viewIndex]);
+  await promiseShown;
+
+  Assert.ok(PanelView.forNode(gPanelViews[viewIndex]).active);
+  assertLabelVisible(viewIndex, true);
+}
+
+/**
+ * Navigates backwards to the specified view, which is displayed as a result.
+ */
+async function goBack(panelIndex, viewIndex) {
+  let promiseShown = BrowserTestUtils.waitForEvent(
+    gPanelViews[viewIndex],
+    "ViewShown"
+  );
+  gPanelMultiViews[panelIndex].goBack();
+  await promiseShown;
+
+  Assert.ok(PanelView.forNode(gPanelViews[viewIndex]).active);
+  assertLabelVisible(viewIndex, true);
+}
+
+/**
+ * Records the specified events on an element into the specified array. An
+ * optional callback can be used to respond to events and trigger nested events.
+ */
+function recordEvents(
+  element,
+  eventTypes,
+  recordArray,
+  eventCallback = () => {}
+) {
+  let nestedEvents = [];
+  element.recorders = eventTypes.map(eventType => {
+    let recorder = {
+      eventType,
+      listener(event) {
+        let eventString =
+          nestedEvents.join("") + `${event.originalTarget.id}: ${event.type}`;
+        info(`Event on ${eventString}`);
+        recordArray.push(eventString);
+        // Any synchronous event triggered from within the given callback will
+        // include information about the current event.
+        nestedEvents.unshift(`${eventString} > `);
+        eventCallback(event);
+        nestedEvents.shift();
+      },
+    };
+    element.addEventListener(recorder.eventType, recorder.listener);
+    return recorder;
+  });
+}
+
+/**
+ * Stops recording events on an element.
+ */
+function stopRecordingEvents(element) {
+  for (let recorder of element.recorders) {
+    element.removeEventListener(recorder.eventType, recorder.listener);
+  }
+  delete element.recorders;
+}
+
+/**
+ * Sets up the elements in the browser window that will be used by all the other
+ * regression tests. Since the panel and view elements can live anywhere in the
+ * document, they are simply added to the same toolbar as the panel anchors.
+ *
+ * <toolbar id="nav-bar">
+ *     <toolbarbutton/>      ->  gPanelAnchors[panelIndex]
+ *     <panel>               ->  gPanels[panelIndex]
+ *       <panelmultiview/>   ->  gPanelMultiViews[panelIndex]
+ *     </panel>
+ *     <panelview>           ->  gPanelViews[viewIndex]
+ *       <label/>            ->  gPanelViewLabels[viewIndex]
+ *     </panelview>
+ * </toolbar>
+ */
+add_task(async function test_setup() {
+  let navBar = document.getElementById("nav-bar");
+
+  for (let i = 0; i < PANELS_COUNT; i++) {
+    gPanelAnchors[i] = document.createXULElement("toolbarbutton");
+    gPanelAnchors[i].classList.add(
+      "toolbarbutton-1",
+      "chromeclass-toolbar-additional"
+    );
+    navBar.appendChild(gPanelAnchors[i]);
+
+    gPanels[i] = document.createXULElement("panel");
+    gPanels[i].id = "panel-" + i;
+    gPanels[i].setAttribute("type", "arrow");
+    gPanels[i].setAttribute("photon", true);
+    navBar.appendChild(gPanels[i]);
+
+    gPanelMultiViews[i] = document.createXULElement("panelmultiview");
+    gPanelMultiViews[i].id = "panelmultiview-" + i;
+    gPanels[i].appendChild(gPanelMultiViews[i]);
+  }
+
+  for (let i = 0; i < PANELVIEWS_COUNT; i++) {
+    gPanelViews[i] = document.createXULElement("panelview");
+    gPanelViews[i].id = "panelview-" + i;
+    navBar.appendChild(gPanelViews[i]);
+
+    gPanelViewLabels[i] = document.createXULElement("label");
+    gPanelViewLabels[i].setAttribute("value", "PanelView " + i);
+    gPanelViews[i].appendChild(gPanelViewLabels[i]);
+  }
+
+  registerCleanupFunction(() => {
+    [...gPanelAnchors, ...gPanels, ...gPanelViews].forEach(e => e.remove());
+  });
+});
+
+/**
+ * Shows and hides all views in a panel with this static structure:
+ *
+ * - Panel 0
+ *   - View 0
+ *     - View 1
+ *     - View 3
+ *       - View 2
+ */
+add_task(async function test_simple() {
+  // Show main view 0.
+  await openPopup(0, 0);
+
+  // Show and hide subview 1.
+  await showSubView(0, 1);
+  assertLabelVisible(0, false);
+  await goBack(0, 0);
+  assertLabelVisible(1, false);
+
+  // Show subview 3.
+  await showSubView(0, 3);
+  assertLabelVisible(0, false);
+
+  // Show and hide subview 2.
+  await showSubView(0, 2);
+  assertLabelVisible(3, false);
+  await goBack(0, 3);
+  assertLabelVisible(2, false);
+
+  // Hide subview 3.
+  await goBack(0, 0);
+  assertLabelVisible(3, false);
+
+  // Hide main view 0.
+  await hidePopup(0);
+  assertLabelVisible(0, false);
+});
+
+/**
+ * Tests the event sequence in a panel with this static structure:
+ *
+ * - Panel 0
+ *   - View 0
+ *     - View 1
+ *     - View 3
+ *       - View 2
+ */
+add_task(async function test_simple_event_sequence() {
+  let recordArray = [];
+  recordEvents(gPanels[0], EVENT_TYPES, recordArray);
+
+  await openPopup(0, 0);
+  await showSubView(0, 1);
+  await goBack(0, 0);
+  await showSubView(0, 3);
+  await showSubView(0, 2);
+  await goBack(0, 3);
+  await goBack(0, 0);
+  await hidePopup(0);
+
+  stopRecordingEvents(gPanels[0]);
+
+  Assert.deepEqual(recordArray, [
+    "panelview-0: ViewShowing",
+    "panelview-0: ViewShown",
+    "panel-0: popupshown",
+    "panelview-1: ViewShowing",
+    "panelview-1: ViewShown",
+    "panelview-1: ViewHiding",
+    "panelview-0: ViewShown",
+    "panelview-3: ViewShowing",
+    "panelview-3: ViewShown",
+    "panelview-2: ViewShowing",
+    "panelview-2: ViewShown",
+    "panelview-2: ViewHiding",
+    "panelview-3: ViewShown",
+    "panelview-3: ViewHiding",
+    "panelview-0: ViewShown",
+    "panelview-0: ViewHiding",
+    "panelmultiview-0: PanelMultiViewHidden",
+    "panel-0: popuphidden",
+  ]);
+});
+
+/**
+ * Tests that further navigation is suppressed until the new view is shown.
+ */
+add_task(async function test_navigation_suppression() {
+  await openPopup(0, 0);
+
+  // Test re-entering the "showSubView" method.
+  let promiseShown = BrowserTestUtils.waitForEvent(gPanelViews[1], "ViewShown");
+  gPanelMultiViews[0].showSubView(gPanelViews[1]);
+  Assert.ok(
+    !PanelView.forNode(gPanelViews[0]).active,
+    "The previous view should become inactive synchronously."
+  );
+
+  // The following call will have no effect.
+  gPanelMultiViews[0].showSubView(gPanelViews[2]);
+  await promiseShown;
+
+  // Test re-entering the "goBack" method.
+  promiseShown = BrowserTestUtils.waitForEvent(gPanelViews[0], "ViewShown");
+  gPanelMultiViews[0].goBack();
+  Assert.ok(
+    !PanelView.forNode(gPanelViews[1]).active,
+    "The previous view should become inactive synchronously."
+  );
+
+  // The following call will have no effect.
+  gPanelMultiViews[0].goBack();
+  await promiseShown;
+
+  // Main view 0 should be displayed.
+  assertLabelVisible(0, true);
+
+  await hidePopup(0);
+});
+
+/**
+ * Tests reusing views that are already open in another panel. In this test, the
+ * structure of the first panel will change dynamically:
+ *
+ * - Panel 0
+ *   - View 0
+ *     - View 1
+ * - Panel 1
+ *   - View 1
+ *     - View 2
+ * - Panel 0
+ *   - View 1
+ *     - View 0
+ */
+add_task(async function test_switch_event_sequence() {
+  let recordArray = [];
+  recordEvents(gPanels[0], EVENT_TYPES, recordArray);
+  recordEvents(gPanels[1], EVENT_TYPES, recordArray);
+
+  // Show panel 0.
+  await openPopup(0, 0);
+  await showSubView(0, 1);
+
+  // Show panel 1 with the view that is already open and visible in panel 0.
+  // This will close panel 0 automatically.
+  await openPopup(1, 1);
+  await showSubView(1, 2);
+
+  // Show panel 0 with a view that is already open but invisible in panel 1.
+  // This will close panel 1 automatically.
+  await openPopup(0, 1);
+  await showSubView(0, 0);
+
+  // Hide panel 0.
+  await hidePopup(0);
+
+  stopRecordingEvents(gPanels[0]);
+  stopRecordingEvents(gPanels[1]);
+
+  Assert.deepEqual(recordArray, [
+    "panelview-0: ViewShowing",
+    "panelview-0: ViewShown",
+    "panel-0: popupshown",
+    "panelview-1: ViewShowing",
+    "panelview-1: ViewShown",
+    "panelview-1: ViewHiding",
+    "panelview-0: ViewHiding",
+    "panelmultiview-0: PanelMultiViewHidden",
+    "panel-0: popuphidden",
+    "panelview-1: ViewShowing",
+    "panel-1: popupshown",
+    "panelview-1: ViewShown",
+    "panelview-2: ViewShowing",
+    "panelview-2: ViewShown",
+    "panel-1: popuphidden",
+    "panelview-2: ViewHiding",
+    "panelview-1: ViewHiding",
+    "panelmultiview-1: PanelMultiViewHidden",
+    "panelview-1: ViewShowing",
+    "panelview-1: ViewShown",
+    "panel-0: popupshown",
+    "panelview-0: ViewShowing",
+    "panelview-0: ViewShown",
+    "panelview-0: ViewHiding",
+    "panelview-1: ViewHiding",
+    "panelmultiview-0: PanelMultiViewHidden",
+    "panel-0: popuphidden",
+  ]);
+});
+
+/**
+ * Tests the event sequence when opening the main view is canceled.
+ */
+add_task(async function test_cancel_mainview_event_sequence() {
+  let recordArray = [];
+  recordEvents(gPanels[0], EVENT_TYPES, recordArray, event => {
+    if (event.type == "ViewShowing") {
+      event.preventDefault();
+    }
+  });
+
+  gPanelMultiViews[0].setAttribute("mainViewId", gPanelViews[0].id);
+
+  let promiseHidden = BrowserTestUtils.waitForEvent(gPanels[0], "popuphidden");
+  PanelMultiView.openPopup(
+    gPanels[0],
+    gPanelAnchors[0],
+    "bottomright topright"
+  );
+  await promiseHidden;
+
+  stopRecordingEvents(gPanels[0]);
+
+  Assert.deepEqual(recordArray, [
+    "panelview-0: ViewShowing",
+    "panelview-0: ViewHiding",
+    "panelmultiview-0: PanelMultiViewHidden",
+    "panelmultiview-0: popuphidden",
+  ]);
+});
+
+/**
+ * Tests the event sequence when opening a subview is canceled.
+ */
+add_task(async function test_cancel_subview_event_sequence() {
+  let recordArray = [];
+  recordEvents(gPanels[0], EVENT_TYPES, recordArray, event => {
+    if (
+      event.type == "ViewShowing" &&
+      event.originalTarget.id == gPanelViews[1].id
+    ) {
+      event.preventDefault();
+    }
+  });
+
+  await openPopup(0, 0);
+
+  let promiseHiding = BrowserTestUtils.waitForEvent(
+    gPanelViews[1],
+    "ViewHiding"
+  );
+  gPanelMultiViews[0].showSubView(gPanelViews[1]);
+  await promiseHiding;
+
+  // Only the subview should have received the hidden event at this point.
+  Assert.deepEqual(recordArray, [
+    "panelview-0: ViewShowing",
+    "panelview-0: ViewShown",
+    "panel-0: popupshown",
+    "panelview-1: ViewShowing",
+    "panelview-1: ViewHiding",
+  ]);
+  recordArray.length = 0;
+
+  await hidePopup(0);
+
+  stopRecordingEvents(gPanels[0]);
+
+  Assert.deepEqual(recordArray, [
+    "panelview-0: ViewHiding",
+    "panelmultiview-0: PanelMultiViewHidden",
+    "panel-0: popuphidden",
+  ]);
+});
+
+/**
+ * Tests the event sequence when closing the panel while opening the main view.
+ */
+add_task(async function test_close_while_showing_mainview_event_sequence() {
+  let recordArray = [];
+  recordEvents(gPanels[0], EVENT_TYPES, recordArray, event => {
+    if (event.type == "ViewShowing") {
+      PanelMultiView.hidePopup(gPanels[0]);
+    }
+  });
+
+  gPanelMultiViews[0].setAttribute("mainViewId", gPanelViews[0].id);
+
+  let promiseHidden = BrowserTestUtils.waitForEvent(gPanels[0], "popuphidden");
+  let promiseHiding = BrowserTestUtils.waitForEvent(
+    gPanelViews[0],
+    "ViewHiding"
+  );
+  PanelMultiView.openPopup(
+    gPanels[0],
+    gPanelAnchors[0],
+    "bottomright topright"
+  );
+  await promiseHiding;
+  await promiseHidden;
+
+  stopRecordingEvents(gPanels[0]);
+
+  Assert.deepEqual(recordArray, [
+    "panelview-0: ViewShowing",
+    "panelview-0: ViewShowing > panelview-0: ViewHiding",
+    "panelview-0: ViewShowing > panelmultiview-0: PanelMultiViewHidden",
+    "panelview-0: ViewShowing > panelmultiview-0: popuphidden",
+  ]);
+});
+
+/**
+ * Tests the event sequence when closing the panel while opening a subview.
+ */
+add_task(async function test_close_while_showing_subview_event_sequence() {
+  let recordArray = [];
+  recordEvents(gPanels[0], EVENT_TYPES, recordArray, event => {
+    if (
+      event.type == "ViewShowing" &&
+      event.originalTarget.id == gPanelViews[1].id
+    ) {
+      PanelMultiView.hidePopup(gPanels[0]);
+    }
+  });
+
+  await openPopup(0, 0);
+
+  let promiseHidden = BrowserTestUtils.waitForEvent(gPanels[0], "popuphidden");
+  gPanelMultiViews[0].showSubView(gPanelViews[1]);
+  await promiseHidden;
+
+  stopRecordingEvents(gPanels[0]);
+
+  Assert.deepEqual(recordArray, [
+    "panelview-0: ViewShowing",
+    "panelview-0: ViewShown",
+    "panel-0: popupshown",
+    "panelview-1: ViewShowing",
+    "panelview-1: ViewShowing > panelview-1: ViewHiding",
+    "panelview-1: ViewShowing > panelview-0: ViewHiding",
+    "panelview-1: ViewShowing > panelmultiview-0: PanelMultiViewHidden",
+    "panelview-1: ViewShowing > panel-0: popuphidden",
+  ]);
+});
diff --git a/browser/components/customizableui/test/browser_PanelMultiView_focus.js b/browser/components/customizableui/test/browser_PanelMultiView_focus.js
new file mode 100644
index 0000000000..869b5cc808
--- /dev/null
+++ b/browser/components/customizableui/test/browser_PanelMultiView_focus.js
@@ -0,0 +1,169 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test the focus behavior when opening PanelViews.
+ */
+
+let gAnchor;
+let gPanel;
+let gPanelMultiView;
+let gMainView;
+let gMainButton;
+let gMainSubButton;
+let gSubView;
+let gSubButton;
+
+function createWith(doc, tag, props) {
+  let el = doc.createXULElement(tag);
+  for (let prop in props) {
+    el.setAttribute(prop, props[prop]);
+  }
+  return el;
+}
+
+add_setup(async function () {
+  let navBar = document.getElementById("nav-bar");
+  gAnchor = document.createXULElement("toolbarbutton");
+  // Must be focusable in order for key presses to work.
+  gAnchor.style["-moz-user-focus"] = "normal";
+  navBar.appendChild(gAnchor);
+  let onPress = event =>
+    PanelMultiView.openPopup(gPanel, gAnchor, {
+      triggerEvent: event,
+    });
+  gAnchor.addEventListener("keypress", onPress);
+  gAnchor.addEventListener("click", onPress);
+  gPanel = document.createXULElement("panel");
+  navBar.appendChild(gPanel);
+  gPanelMultiView = document.createXULElement("panelmultiview");
+  gPanelMultiView.setAttribute("mainViewId", "testMainView");
+  gPanel.appendChild(gPanelMultiView);
+
+  gMainView = document.createXULElement("panelview");
+  gMainView.id = "testMainView";
+  gPanelMultiView.appendChild(gMainView);
+  gMainButton = createWith(document, "button", { label: "gMainButton" });
+  gMainView.appendChild(gMainButton);
+  gMainSubButton = createWith(document, "button", { label: "gMainSubButton" });
+  gMainView.appendChild(gMainSubButton);
+  gMainSubButton.addEventListener("command", () =>
+    gPanelMultiView.showSubView("testSubView", gMainSubButton)
+  );
+
+  gSubView = document.createXULElement("panelview");
+  gSubView.id = "testSubView";
+  gPanelMultiView.appendChild(gSubView);
+  gSubButton = createWith(document, "button", { label: "gSubButton" });
+  gSubView.appendChild(gSubButton);
+
+  registerCleanupFunction(() => {
+    gAnchor.remove();
+    gPanel.remove();
+  });
+});
+
+// Activate the main view by pressing a key. Focus should be moved inside.
+add_task(async function testMainViewByKeypress() {
+  gAnchor.focus();
+  await gCUITestUtils.openPanelMultiView(gPanel, gMainView, () =>
+    EventUtils.synthesizeKey(" ")
+  );
+  Assert.equal(
+    document.activeElement,
+    gMainButton,
+    "Focus on button in main view"
+  );
+  await gCUITestUtils.hidePanelMultiView(gPanel, () =>
+    PanelMultiView.hidePopup(gPanel)
+  );
+});
+
+// Activate the main view by clicking the mouse. Focus should not be moved
+// inside.
+add_task(async function testMainViewByClick() {
+  await gCUITestUtils.openPanelMultiView(gPanel, gMainView, () =>
+    gAnchor.click()
+  );
+  Assert.notEqual(
+    document.activeElement,
+    gMainButton,
+    "Focus not on button in main view"
+  );
+  await gCUITestUtils.hidePanelMultiView(gPanel, () =>
+    PanelMultiView.hidePopup(gPanel)
+  );
+});
+
+// Activate the subview by pressing a key. Focus should be moved to the first
+// button after the Back button.
+add_task(async function testSubViewByKeypress() {
+  await gCUITestUtils.openPanelMultiView(gPanel, gMainView, () =>
+    gAnchor.click()
+  );
+  while (document.activeElement != gMainSubButton) {
+    EventUtils.synthesizeKey("KEY_Tab", { shiftKey: true });
+  }
+  let shown = BrowserTestUtils.waitForEvent(gSubView, "ViewShown");
+  EventUtils.synthesizeKey(" ");
+  await shown;
+  Assert.equal(
+    document.activeElement,
+    gSubButton,
+    "Focus on first button after Back button in subview"
+  );
+  await gCUITestUtils.hidePanelMultiView(gPanel, () =>
+    PanelMultiView.hidePopup(gPanel)
+  );
+});
+
+// Activate the subview by clicking the mouse. Focus should not be moved
+// inside.
+add_task(async function testSubViewByClick() {
+  await gCUITestUtils.openPanelMultiView(gPanel, gMainView, () =>
+    gAnchor.click()
+  );
+  let shown = BrowserTestUtils.waitForEvent(gSubView, "ViewShown");
+  gMainSubButton.click();
+  await shown;
+  let backButton = gSubView.querySelector(".subviewbutton-back");
+  Assert.notEqual(
+    document.activeElement,
+    backButton,
+    "Focus not on Back button in subview"
+  );
+  Assert.notEqual(
+    document.activeElement,
+    gSubButton,
+    "Focus not on button after Back button in subview"
+  );
+  await gCUITestUtils.hidePanelMultiView(gPanel, () =>
+    PanelMultiView.hidePopup(gPanel)
+  );
+});
+
+// Test that focus is restored when going back to a previous view.
+add_task(async function testBackRestoresFocus() {
+  await gCUITestUtils.openPanelMultiView(gPanel, gMainView, () =>
+    gAnchor.click()
+  );
+  while (document.activeElement != gMainSubButton) {
+    EventUtils.synthesizeKey("KEY_Tab", { shiftKey: true });
+  }
+  let shown = BrowserTestUtils.waitForEvent(gSubView, "ViewShown");
+  EventUtils.synthesizeKey(" ");
+  await shown;
+  shown = BrowserTestUtils.waitForEvent(gMainView, "ViewShown");
+  EventUtils.synthesizeKey("KEY_ArrowLeft");
+  await shown;
+  Assert.equal(
+    document.activeElement,
+    gMainSubButton,
+    "Focus on sub button in main view"
+  );
+  await gCUITestUtils.hidePanelMultiView(gPanel, () =>
+    PanelMultiView.hidePopup(gPanel)
+  );
+});
diff --git a/browser/components/customizableui/test/browser_PanelMultiView_keyboard.js b/browser/components/customizableui/test/browser_PanelMultiView_keyboard.js
new file mode 100644
index 0000000000..baaa38c224
--- /dev/null
+++ b/browser/components/customizableui/test/browser_PanelMultiView_keyboard.js
@@ -0,0 +1,582 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test the keyboard behavior of PanelViews.
+ */
+
+const kEmbeddedDocUrl =
+  'data:text/html,<textarea id="docTextarea">value</textarea><button id="docButton"></button>';
+
+let gAnchor;
+let gPanel;
+let gPanelMultiView;
+let gMainView;
+let gMainContext;
+let gMainButton1;
+let gMainMenulist;
+let gMainRadiogroup;
+let gMainTextbox;
+let gMainButton2;
+let gMainButton3;
+let gCheckbox;
+let gNamespacedLink;
+let gLink;
+let gMainTabOrder;
+let gMainArrowOrder;
+let gSubView;
+let gSubButton;
+let gSubTextarea;
+let gBrowserView;
+let gBrowserBrowser;
+let gIframeView;
+let gIframeIframe;
+let gToggle;
+
+async function openPopup() {
+  let shown = BrowserTestUtils.waitForEvent(gMainView, "ViewShown");
+  PanelMultiView.openPopup(gPanel, gAnchor, "bottomright topright");
+  await shown;
+}
+
+async function hidePopup() {
+  let hidden = BrowserTestUtils.waitForEvent(gPanel, "popuphidden");
+  PanelMultiView.hidePopup(gPanel);
+  await hidden;
+}
+
+async function showSubView(view = gSubView) {
+  let shown = BrowserTestUtils.waitForEvent(view, "ViewShown");
+  // We must show with an anchor so the Back button is generated.
+  gPanelMultiView.showSubView(view, gMainButton1);
+  await shown;
+}
+
+async function expectFocusAfterKey(aKey, aFocus) {
+  let res = aKey.match(/^(Shift\+)?(.+)$/);
+  let shift = Boolean(res[1]);
+  let key;
+  if (res[2].length == 1) {
+    key = res[2]; // Character.
+  } else {
+    key = "KEY_" + res[2]; // Tab, ArrowRight, etc.
+  }
+  info("Waiting for focus on " + aFocus.id);
+  let focused = BrowserTestUtils.waitForEvent(aFocus, "focus");
+  EventUtils.synthesizeKey(key, { shiftKey: shift });
+  await focused;
+  ok(true, aFocus.id + " focused after " + aKey + " pressed");
+}
+
+add_setup(async function () {
+  // This shouldn't be necessary - but it is, because we use same-process frames.
+  // https://bugzilla.mozilla.org/show_bug.cgi?id=1565276 covers improving this.
+  await SpecialPowers.pushPrefEnv({
+    set: [["security.allow_unsafe_parent_loads", true]],
+  });
+  let navBar = document.getElementById("nav-bar");
+  gAnchor = document.createXULElement("toolbarbutton");
+  navBar.appendChild(gAnchor);
+  gPanel = document.createXULElement("panel");
+  navBar.appendChild(gPanel);
+  gPanelMultiView = document.createXULElement("panelmultiview");
+  gPanelMultiView.setAttribute("mainViewId", "testMainView");
+  gPanel.appendChild(gPanelMultiView);
+
+  gMainView = document.createXULElement("panelview");
+  gMainView.id = "testMainView";
+  gPanelMultiView.appendChild(gMainView);
+  gMainContext = document.createXULElement("menupopup");
+  gMainContext.id = "gMainContext";
+  gMainView.appendChild(gMainContext);
+  gMainContext.appendChild(document.createXULElement("menuitem"));
+  gMainButton1 = document.createXULElement("button");
+  gMainButton1.id = "gMainButton1";
+  gMainView.appendChild(gMainButton1);
+  // We use this for anchoring subviews, so it must have a label.
+  gMainButton1.setAttribute("label", "gMainButton1");
+  gMainButton1.setAttribute("context", "gMainContext");
+  gMainMenulist = document.createXULElement("menulist");
+  gMainMenulist.id = "gMainMenulist";
+  gMainView.appendChild(gMainMenulist);
+  let menuPopup = document.createXULElement("menupopup");
+  gMainMenulist.appendChild(menuPopup);
+  let item = document.createXULElement("menuitem");
+  item.setAttribute("value", "1");
+  item.setAttribute("selected", "true");
+  menuPopup.appendChild(item);
+  item = document.createXULElement("menuitem");
+  item.setAttribute("value", "2");
+  menuPopup.appendChild(item);
+  gMainRadiogroup = document.createXULElement("radiogroup");
+  gMainRadiogroup.id = "gMainRadiogroup";
+  gMainView.appendChild(gMainRadiogroup);
+  let radio = document.createXULElement("radio");
+  radio.setAttribute("value", "1");
+  radio.setAttribute("selected", "true");
+  gMainRadiogroup.appendChild(radio);
+  radio = document.createXULElement("radio");
+  radio.setAttribute("value", "2");
+  gMainRadiogroup.appendChild(radio);
+  gMainTextbox = document.createElement("input");
+  gMainTextbox.id = "gMainTextbox";
+  gMainView.appendChild(gMainTextbox);
+  gMainTextbox.setAttribute("value", "value");
+  gMainButton2 = document.createXULElement("button");
+  gMainButton2.id = "gMainButton2";
+  gMainView.appendChild(gMainButton2);
+  gMainButton3 = document.createXULElement("button");
+  gMainButton3.id = "gMainButton3";
+  gMainView.appendChild(gMainButton3);
+  gCheckbox = document.createXULElement("checkbox");
+  gCheckbox.id = "gCheckbox";
+  gMainView.appendChild(gCheckbox);
+
+  // moz-support-links in XUL documents are created with the
+  // <html:a> tag and so we need to test this separately from
+  // <a> tags.
+  gNamespacedLink = document.createElementNS(
+    "http://www.w3.org/1999/xhtml",
+    "html:a"
+  );
+  gNamespacedLink.href = "www.mozilla.org";
+  gNamespacedLink.innerText = "gNamespacedLink";
+  gNamespacedLink.id = "gNamespacedLink";
+  gMainView.appendChild(gNamespacedLink);
+  gLink = document.createElement("a");
+  gLink.href = "www.mozilla.org";
+  gLink.innerText = "gLink";
+  gLink.id = "gLink";
+  gMainView.appendChild(gLink);
+  await window.ensureCustomElements("moz-toggle");
+  gToggle = document.createElement("moz-toggle");
+  gMainView.appendChild(gToggle);
+
+  gMainTabOrder = [
+    gMainButton1,
+    gMainMenulist,
+    gMainRadiogroup,
+    gMainTextbox,
+    gMainButton2,
+    gMainButton3,
+    gCheckbox,
+    gNamespacedLink,
+    gLink,
+    gToggle,
+  ];
+  gMainArrowOrder = [
+    gMainButton1,
+    gMainButton2,
+    gMainButton3,
+    gCheckbox,
+    gNamespacedLink,
+    gLink,
+    gToggle,
+  ];
+
+  gSubView = document.createXULElement("panelview");
+  gSubView.id = "testSubView";
+  gPanelMultiView.appendChild(gSubView);
+  gSubButton = document.createXULElement("button");
+  gSubView.appendChild(gSubButton);
+  gSubTextarea = document.createElementNS(
+    "http://www.w3.org/1999/xhtml",
+    "textarea"
+  );
+  gSubTextarea.id = "gSubTextarea";
+  gSubView.appendChild(gSubTextarea);
+  gSubTextarea.value = "value";
+
+  gBrowserView = document.createXULElement("panelview");
+  gBrowserView.id = "testBrowserView";
+  gPanelMultiView.appendChild(gBrowserView);
+  gBrowserBrowser = document.createXULElement("browser");
+  gBrowserBrowser.id = "GBrowserBrowser";
+  gBrowserBrowser.setAttribute("type", "content");
+  gBrowserBrowser.setAttribute("src", kEmbeddedDocUrl);
+  gBrowserBrowser.style.minWidth = gBrowserBrowser.style.minHeight = "100px";
+  gBrowserView.appendChild(gBrowserBrowser);
+
+  gIframeView = document.createXULElement("panelview");
+  gIframeView.id = "testIframeView";
+  gPanelMultiView.appendChild(gIframeView);
+  gIframeIframe = document.createXULElement("iframe");
+  gIframeIframe.id = "gIframeIframe";
+  gIframeIframe.setAttribute("src", kEmbeddedDocUrl);
+  gIframeView.appendChild(gIframeIframe);
+
+  registerCleanupFunction(() => {
+    gAnchor.remove();
+    gPanel.remove();
+  });
+});
+
+// Test that the tab key focuses all expected controls.
+add_task(async function testTab() {
+  await openPopup();
+  for (let elem of gMainTabOrder) {
+    await expectFocusAfterKey("Tab", elem);
+  }
+  // Wrap around.
+  await expectFocusAfterKey("Tab", gMainTabOrder[0]);
+  await hidePopup();
+});
+
+// Test that the shift+tab key focuses all expected controls.
+add_task(async function testShiftTab() {
+  await openPopup();
+  for (let i = gMainTabOrder.length - 1; i >= 0; --i) {
+    await expectFocusAfterKey("Shift+Tab", gMainTabOrder[i]);
+  }
+  // Wrap around.
+  await expectFocusAfterKey(
+    "Shift+Tab",
+    gMainTabOrder[gMainTabOrder.length - 1]
+  );
+  await hidePopup();
+});
+
+// Test that the down arrow key skips menulists and textboxes.
+add_task(async function testDownArrow() {
+  await openPopup();
+  for (let elem of gMainArrowOrder) {
+    await expectFocusAfterKey("ArrowDown", elem);
+  }
+  // Wrap around.
+  await expectFocusAfterKey("ArrowDown", gMainArrowOrder[0]);
+  await hidePopup();
+});
+
+// Test that the up arrow key skips menulists and textboxes.
+add_task(async function testUpArrow() {
+  await openPopup();
+  for (let i = gMainArrowOrder.length - 1; i >= 0; --i) {
+    await expectFocusAfterKey("ArrowUp", gMainArrowOrder[i]);
+  }
+  // Wrap around.
+  await expectFocusAfterKey(
+    "ArrowUp",
+    gMainArrowOrder[gMainArrowOrder.length - 1]
+  );
+  await hidePopup();
+});
+
+// Test that the home/end keys move to the first/last controls.
+add_task(async function testHomeEnd() {
+  await openPopup();
+  await expectFocusAfterKey("Home", gMainArrowOrder[0]);
+  await expectFocusAfterKey("End", gMainArrowOrder[gMainArrowOrder.length - 1]);
+  await hidePopup();
+});
+
+// Test that the up/down arrow keys work as expected in menulists.
+add_task(async function testArrowsMenulist() {
+  await openPopup();
+  gMainMenulist.focus();
+  is(document.activeElement, gMainMenulist, "menulist focused");
+  is(gMainMenulist.value, "1", "menulist initial value 1");
+  if (AppConstants.platform == "macosx") {
+    // On Mac, down/up arrows just open the menulist.
+    let popup = gMainMenulist.menupopup;
+    for (let key of ["ArrowDown", "ArrowUp"]) {
+      let shown = BrowserTestUtils.waitForEvent(popup, "popupshown");
+      EventUtils.synthesizeKey("KEY_" + key);
+      await shown;
+      ok(gMainMenulist.open, "menulist open after " + key);
+      let hidden = BrowserTestUtils.waitForEvent(popup, "popuphidden");
+      EventUtils.synthesizeKey("KEY_Escape");
+      await hidden;
+      ok(!gMainMenulist.open, "menulist closed after Escape");
+    }
+  } else {
+    // On other platforms, down/up arrows change the value without opening the
+    // menulist.
+    EventUtils.synthesizeKey("KEY_ArrowDown");
+    is(
+      document.activeElement,
+      gMainMenulist,
+      "menulist still focused after ArrowDown"
+    );
+    is(gMainMenulist.value, "2", "menulist value 2 after ArrowDown");
+    EventUtils.synthesizeKey("KEY_ArrowUp");
+    is(
+      document.activeElement,
+      gMainMenulist,
+      "menulist still focused after ArrowUp"
+    );
+    is(gMainMenulist.value, "1", "menulist value 1 after ArrowUp");
+  }
+  await hidePopup();
+});
+
+// Test that the tab key closes an open menu list.
+add_task(async function testTabOpenMenulist() {
+  await openPopup();
+  gMainMenulist.focus();
+  is(document.activeElement, gMainMenulist, "menulist focused");
+  let popup = gMainMenulist.menupopup;
+  let shown = BrowserTestUtils.waitForEvent(popup, "popupshown");
+  gMainMenulist.open = true;
+  await shown;
+  ok(gMainMenulist.open, "menulist open");
+  let menuHidden = BrowserTestUtils.waitForEvent(popup, "popuphidden");
+  EventUtils.synthesizeKey("KEY_Tab");
+  await menuHidden;
+  ok(!gMainMenulist.open, "menulist closed after Tab");
+  is(gPanel.state, "open", "Panel should be open");
+  await hidePopup();
+});
+
+if (AppConstants.platform == "macosx") {
+  // Test that using the mouse to open a menulist still allows keyboard navigation
+  // inside it.
+  add_task(async function testNavigateMouseOpenedMenulist() {
+    await openPopup();
+    let popup = gMainMenulist.menupopup;
+    let shown = BrowserTestUtils.waitForEvent(popup, "popupshown");
+    gMainMenulist.open = true;
+    await shown;
+    ok(gMainMenulist.open, "menulist open");
+    let oldFocus = document.activeElement;
+    let oldSelectedItem = gMainMenulist.selectedItem;
+    ok(
+      oldSelectedItem.hasAttribute("_moz-menuactive"),
+      "Selected item should show up as active"
+    );
+    EventUtils.synthesizeKey("KEY_ArrowDown");
+    await TestUtils.waitForCondition(
+      () => !oldSelectedItem.hasAttribute("_moz-menuactive")
+    );
+    is(oldFocus, document.activeElement, "Focus should not move on mac");
+    ok(
+      !oldSelectedItem.hasAttribute("_moz-menuactive"),
+      "Selected item should change"
+    );
+
+    let menuHidden = BrowserTestUtils.waitForEvent(popup, "popuphidden");
+    EventUtils.synthesizeKey("KEY_Tab");
+    await menuHidden;
+    ok(!gMainMenulist.open, "menulist closed after Tab");
+    is(gPanel.state, "open", "Panel should be open");
+    await hidePopup();
+  });
+}
+
+// Test that the up/down arrow keys work as expected in radiogroups.
+add_task(async function testArrowsRadiogroup() {
+  await openPopup();
+  gMainRadiogroup.focus();
+  is(document.activeElement, gMainRadiogroup, "radiogroup focused");
+  is(gMainRadiogroup.value, "1", "radiogroup initial value 1");
+  EventUtils.synthesizeKey("KEY_ArrowDown");
+  is(
+    document.activeElement,
+    gMainRadiogroup,
+    "radiogroup still focused after ArrowDown"
+  );
+  is(gMainRadiogroup.value, "2", "radiogroup value 2 after ArrowDown");
+  EventUtils.synthesizeKey("KEY_ArrowUp");
+  is(
+    document.activeElement,
+    gMainRadiogroup,
+    "radiogroup still focused after ArrowUp"
+  );
+  is(gMainRadiogroup.value, "1", "radiogroup value 1 after ArrowUp");
+  await hidePopup();
+});
+
+// Test that pressing space in a textbox inserts a space (instead of trying to
+// activate the control).
+add_task(async function testSpaceTextbox() {
+  await openPopup();
+  gMainTextbox.focus();
+  gMainTextbox.selectionStart = gMainTextbox.selectionEnd = 0;
+  EventUtils.synthesizeKey(" ");
+  is(gMainTextbox.value, " value", "Space typed into textbox");
+  gMainTextbox.value = "value";
+  await hidePopup();
+});
+
+// Tests that the left arrow key normally moves back to the previous view.
+add_task(async function testLeftArrow() {
+  await openPopup();
+  await showSubView();
+  let shown = BrowserTestUtils.waitForEvent(gMainView, "ViewShown");
+  EventUtils.synthesizeKey("KEY_ArrowLeft");
+  await shown;
+  ok("Moved to previous view after ArrowLeft");
+  await hidePopup();
+});
+
+// Tests that the left arrow key moves the caret in a textarea in a subview
+// (instead of going back to the previous view).
+add_task(async function testLeftArrowTextarea() {
+  await openPopup();
+  await showSubView();
+  gSubTextarea.focus();
+  is(document.activeElement, gSubTextarea, "textarea focused");
+  EventUtils.synthesizeKey("KEY_End");
+  is(gSubTextarea.selectionStart, 5, "selectionStart 5 after End");
+  EventUtils.synthesizeKey("KEY_ArrowLeft");
+  is(gSubTextarea.selectionStart, 4, "selectionStart 4 after ArrowLeft");
+  is(document.activeElement, gSubTextarea, "textarea still focused");
+  await hidePopup();
+});
+
+// Test navigation to a button which is initially disabled and later enabled.
+add_task(async function testDynamicButton() {
+  gMainButton2.disabled = true;
+  await openPopup();
+  await expectFocusAfterKey("ArrowDown", gMainButton1);
+  await expectFocusAfterKey("ArrowDown", gMainButton3);
+  gMainButton2.disabled = false;
+  await expectFocusAfterKey("ArrowUp", gMainButton2);
+  await hidePopup();
+});
+
+add_task(async function testActivation() {
+  function checkActivated(elem, activationFn, reason) {
+    let activated = false;
+    elem.onclick = function () {
+      activated = true;
+    };
+    activationFn();
+    ok(activated, "Should have activated button after " + reason);
+    elem.onclick = null;
+  }
+  await openPopup();
+  await expectFocusAfterKey("ArrowDown", gMainButton1);
+  checkActivated(
+    gMainButton1,
+    () => EventUtils.synthesizeKey("KEY_Enter"),
+    "pressing enter"
+  );
+  checkActivated(
+    gMainButton1,
+    () => EventUtils.synthesizeKey(" "),
+    "pressing space"
+  );
+  checkActivated(
+    gMainButton1,
+    () => EventUtils.synthesizeKey("KEY_Enter", { code: "NumpadEnter" }),
+    "pressing numpad enter"
+  );
+  await hidePopup();
+});
+
+// Test that keyboard activation works for buttons responding to mousedown
+// events (instead of command or click). The Library button does this, for
+// example.
+add_task(async function testActivationMousedown() {
+  await openPopup();
+  await expectFocusAfterKey("ArrowDown", gMainButton1);
+  let activated = false;
+  gMainButton1.onmousedown = function () {
+    activated = true;
+  };
+  EventUtils.synthesizeKey(" ");
+  ok(activated, "mousedown activated after space");
+  gMainButton1.onmousedown = null;
+  await hidePopup();
+});
+
+// Test that tab and the arrow keys aren't overridden in embedded documents.
+async function testTabArrowsEmbeddedDoc(aView, aEmbedder) {
+  await openPopup();
+  await showSubView(aView);
+  let doc = aEmbedder.contentDocument;
+  if (doc.readyState != "complete" || doc.location.href != kEmbeddedDocUrl) {
+    info(`Embedded doc readyState ${doc.readyState}, location ${doc.location}`);
+    info("Waiting for load on embedder");
+    // Browsers don't fire load events, and iframes don't fire load events in
+    // typeChrome windows. We can handle both by using a capturing event
+    // listener to capture the load event from the child document.
+    await BrowserTestUtils.waitForEvent(aEmbedder, "load", true);
+    // The original doc might have been a temporary about:blank, so fetch it
+    // again.
+    doc = aEmbedder.contentDocument;
+  }
+  is(doc.location.href, kEmbeddedDocUrl, "Embedded doc has correct URl");
+  let backButton = aView.querySelector(".subviewbutton-back");
+  backButton.id = "docBack";
+  await expectFocusAfterKey("Tab", backButton);
+  // Documents don't have an id property, but expectFocusAfterKey wants one.
+  doc.id = "doc";
+  await expectFocusAfterKey("Tab", doc);
+  // Make sure tab/arrows aren't overridden within the embedded document.
+  let textarea = doc.getElementById("docTextarea");
+  // Tab should really focus the textarea, but default tab handling seems to
+  // skip everything inside the embedder element when run in this test. This
+  // behaves as expected in real panels, though. Force focus to the textarea
+  // and then test from there.
+  textarea.focus();
+  is(doc.activeElement, textarea, "textarea focused");
+  is(textarea.selectionStart, 0, "selectionStart initially 0");
+  EventUtils.synthesizeKey("KEY_ArrowRight");
+  is(textarea.selectionStart, 1, "selectionStart 1 after ArrowRight");
+  EventUtils.synthesizeKey("KEY_ArrowLeft");
+  is(textarea.selectionStart, 0, "selectionStart 0 after ArrowLeft");
+  is(doc.activeElement, textarea, "textarea still focused");
+  let docButton = doc.getElementById("docButton");
+  await expectFocusAfterKey("Tab", docButton);
+  await hidePopup();
+}
+
+// Test that tab and the arrow keys aren't overridden in embedded browsers.
+add_task(async function testTabArrowsBrowser() {
+  await testTabArrowsEmbeddedDoc(gBrowserView, gBrowserBrowser);
+});
+
+// Test that tab and the arrow keys aren't overridden in embedded iframes.
+add_task(async function testTabArrowsIframe() {
+  await testTabArrowsEmbeddedDoc(gIframeView, gIframeIframe);
+});
+
+// Test that the arrow keys aren't overridden in context menus.
+add_task(async function testArowsContext() {
+  await openPopup();
+  await expectFocusAfterKey("ArrowDown", gMainButton1);
+  let shown = BrowserTestUtils.waitForEvent(gMainContext, "popupshown");
+  // There's no cross-platform way to open a context menu from the keyboard.
+  gMainContext.openPopup(gMainButton1);
+  await shown;
+  let item = gMainContext.children[0];
+  ok(
+    !item.getAttribute("_moz-menuactive"),
+    "First context menu item initially inactive"
+  );
+  let active = BrowserTestUtils.waitForEvent(item, "DOMMenuItemActive");
+  EventUtils.synthesizeKey("KEY_ArrowDown");
+  await active;
+  ok(
+    item.getAttribute("_moz-menuactive"),
+    "First context menu item active after ArrowDown"
+  );
+  is(
+    document.activeElement,
+    gMainButton1,
+    "gMainButton1 still focused after ArrowDown"
+  );
+  let hidden = BrowserTestUtils.waitForEvent(gMainContext, "popuphidden");
+  gMainContext.hidePopup();
+  await hidden;
+  await hidePopup();
+});
+
+add_task(async function testMozToggle() {
+  await openPopup();
+  is(gToggle.pressed, false, "The toggle is not pressed initially.");
+  // Focus the toggle via keyboard navigation.
+  while (document.activeElement !== gToggle) {
+    EventUtils.synthesizeKey("KEY_Tab");
+  }
+  EventUtils.synthesizeKey(" ");
+  await gToggle.updateComplete;
+  is(gToggle.pressed, true, "Toggle pressed state changes via spacebar.");
+  EventUtils.synthesizeKey("KEY_Enter");
+  await gToggle.updateComplete;
+  is(gToggle.pressed, false, "Toggle pressed state changes via enter.");
+  await hidePopup();
+});
diff --git a/browser/components/customizableui/test/browser_addons_area.js b/browser/components/customizableui/test/browser_addons_area.js
new file mode 100644
index 0000000000..533d48b238
--- /dev/null
+++ b/browser/components/customizableui/test/browser_addons_area.js
@@ -0,0 +1,76 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Test that widgets provided by extensions can be added to the
+ * ADDONS area, but all other widgets cannot.
+ */
+add_task(async function test_only_extension_widgets_in_addons_area() {
+  registerCleanupFunction(async () => {
+    await CustomizableUI.reset();
+  });
+
+  Assert.ok(
+    !CustomizableUI.canWidgetMoveToArea(
+      "home-button",
+      CustomizableUI.AREA_ADDONS
+    ),
+    "Cannot move a built-in button to the ADDONS area."
+  );
+
+  // Now double-check that we cannot accidentally default a non-extension
+  // widget into the ADDONS area.
+  const kTestDynamicWidget = "a-test-widget";
+  CustomizableUI.createWidget({
+    id: kTestDynamicWidget,
+    label: "Test widget",
+    defaultArea: CustomizableUI.AREA_ADDONS,
+  });
+  Assert.equal(
+    CustomizableUI.getPlacementOfWidget(kTestDynamicWidget),
+    null,
+    "An attempt to put a non-extension widget into the ADDONS area by default should fail."
+  );
+  CustomizableUI.destroyWidget(kTestDynamicWidget);
+
+  const kWebExtensionButtonID1 = "a-test-extension-button";
+
+  CustomizableUI.createWidget({
+    id: kWebExtensionButtonID1,
+    label: "Test extension widget",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+    webExtension: true,
+  });
+
+  Assert.ok(
+    CustomizableUI.canWidgetMoveToArea(
+      kWebExtensionButtonID1,
+      CustomizableUI.AREA_ADDONS
+    ),
+    "Can move extension button to the addons area."
+  );
+
+  CustomizableUI.destroyWidget(kWebExtensionButtonID1);
+
+  // Now check that extension buttons can default to the ADDONS area, if need
+  // be.
+
+  const kWebExtensionButtonID2 = "a-test-extension-button-2";
+
+  CustomizableUI.createWidget({
+    id: kWebExtensionButtonID2,
+    label: "Test extension widget 2",
+    defaultArea: CustomizableUI.AREA_ADDONS,
+    webExtension: true,
+  });
+
+  Assert.equal(
+    CustomizableUI.getPlacementOfWidget(kWebExtensionButtonID2)?.area,
+    CustomizableUI.AREA_ADDONS,
+    "An attempt to put an extension widget into the ADDONS area by default should work."
+  );
+
+  CustomizableUI.destroyWidget(kWebExtensionButtonID2);
+});
diff --git a/browser/components/customizableui/test/browser_allow_dragging_removable_false.js b/browser/components/customizableui/test/browser_allow_dragging_removable_false.js
new file mode 100644
index 0000000000..76269f44ae
--- /dev/null
+++ b/browser/components/customizableui/test/browser_allow_dragging_removable_false.js
@@ -0,0 +1,42 @@
+"use strict";
+
+/**
+ * Test dragging a removable=false widget within its own area as well as to the palette.
+ */
+add_task(async function () {
+  await startCustomizing();
+  let forwardButton = document.getElementById("forward-button");
+  is(
+    forwardButton.getAttribute("removable"),
+    "false",
+    "forward-button should not be removable"
+  );
+  ok(CustomizableUI.inDefaultState, "Should start in default state.");
+
+  let urlbarContainer = document.getElementById("urlbar-container");
+  let placementsAfterDrag = getAreaWidgetIds(CustomizableUI.AREA_NAVBAR);
+  placementsAfterDrag.splice(placementsAfterDrag.indexOf("forward-button"), 1);
+  placementsAfterDrag.splice(
+    placementsAfterDrag.indexOf("urlbar-container"),
+    0,
+    "forward-button"
+  );
+
+  // Force layout flush to ensure the drag completes as expected
+  urlbarContainer.clientWidth;
+
+  simulateItemDrag(forwardButton, urlbarContainer, "start");
+  assertAreaPlacements(CustomizableUI.AREA_NAVBAR, placementsAfterDrag);
+  ok(!CustomizableUI.inDefaultState, "Should no longer be in default state.");
+  let palette = document.getElementById("customization-palette");
+  simulateItemDrag(forwardButton, palette);
+  is(
+    CustomizableUI.getPlacementOfWidget("forward-button").area,
+    CustomizableUI.AREA_NAVBAR,
+    "forward-button was not able to move to palette"
+  );
+
+  await endCustomizing();
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "Should be in default state again.");
+});
diff --git a/browser/components/customizableui/test/browser_backfwd_enabled_post_customize.js b/browser/components/customizableui/test/browser_backfwd_enabled_post_customize.js
new file mode 100644
index 0000000000..e92a363dee
--- /dev/null
+++ b/browser/components/customizableui/test/browser_backfwd_enabled_post_customize.js
@@ -0,0 +1,79 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "http://example.com"
+);
+
+/**
+ * Back/fwd buttons should be re-enabled after customizing.
+ */
+add_task(async function test_back_forward_buttons() {
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.navigation.requireUserInteraction", false]],
+  });
+  let tab = await BrowserTestUtils.openNewForegroundTab(gBrowser, TEST_PATH);
+  let loaded = BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  BrowserTestUtils.loadURIString(
+    tab.linkedBrowser,
+    "data:text/html,A separate page"
+  );
+  await loaded;
+  loaded = BrowserTestUtils.browserLoaded(tab.linkedBrowser);
+  BrowserTestUtils.loadURIString(
+    tab.linkedBrowser,
+    "data:text/html,Another separate page"
+  );
+  await loaded;
+  gBrowser.goBack();
+  await BrowserTestUtils.waitForCondition(() => gBrowser.canGoForward);
+
+  let backButton = document.getElementById("back-button");
+  let forwardButton = document.getElementById("forward-button");
+
+  await BrowserTestUtils.waitForCondition(
+    () =>
+      !backButton.hasAttribute("disabled") &&
+      !forwardButton.hasAttribute("disabled")
+  );
+
+  ok(!backButton.hasAttribute("disabled"), "Back button shouldn't be disabled");
+  ok(
+    !forwardButton.hasAttribute("disabled"),
+    "Forward button shouldn't be disabled"
+  );
+  await startCustomizing();
+
+  is(
+    backButton.getAttribute("disabled"),
+    "true",
+    "Back button should be disabled in customize mode"
+  );
+  is(
+    forwardButton.getAttribute("disabled"),
+    "true",
+    "Forward button should be disabled in customize mode"
+  );
+
+  await endCustomizing();
+
+  await BrowserTestUtils.waitForCondition(
+    () =>
+      !backButton.hasAttribute("disabled") &&
+      !forwardButton.hasAttribute("disabled")
+  );
+
+  ok(
+    !backButton.hasAttribute("disabled"),
+    "Back button shouldn't be disabled after customize mode"
+  );
+  ok(
+    !forwardButton.hasAttribute("disabled"),
+    "Forward button shouldn't be disabled after customize mode"
+  );
+
+  BrowserTestUtils.removeTab(tab);
+});
diff --git a/browser/components/customizableui/test/browser_bookmarks_empty_message.js b/browser/components/customizableui/test/browser_bookmarks_empty_message.js
new file mode 100644
index 0000000000..d4bcba1b27
--- /dev/null
+++ b/browser/components/customizableui/test/browser_bookmarks_empty_message.js
@@ -0,0 +1,35 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function empty_message_on_non_empty_bookmarks_toolbar() {
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "Default state to begin");
+
+  await SpecialPowers.pushPrefEnv({
+    set: [["browser.toolbars.bookmarks.visibility", "always"]],
+  });
+
+  CustomizableUI.removeWidgetFromArea("import-button");
+  CustomizableUI.removeWidgetFromArea("personal-bookmarks");
+  CustomizableUI.addWidgetToArea(
+    "bookmarks-menu-button",
+    CustomizableUI.AREA_BOOKMARKS,
+    0
+  );
+
+  let newWin = await BrowserTestUtils.openNewBrowserWindow();
+  let doc = newWin.document;
+  ok(
+    BrowserTestUtils.is_visible(doc.getElementById("PersonalToolbar")),
+    "Personal toolbar should be visible"
+  );
+  ok(
+    doc.getElementById("personal-toolbar-empty").hidden,
+    "Empty message should be hidden"
+  );
+
+  await BrowserTestUtils.closeWindow(newWin);
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_bookmarks_toolbar_collapsed_restore_default.js b/browser/components/customizableui/test/browser_bookmarks_toolbar_collapsed_restore_default.js
new file mode 100644
index 0000000000..84ddc37d29
--- /dev/null
+++ b/browser/components/customizableui/test/browser_bookmarks_toolbar_collapsed_restore_default.js
@@ -0,0 +1,35 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+"use strict";
+
+// Restoring default should set Bookmarks Toolbar back to "newtab"
+add_task(async function () {
+  let prefName = "browser.toolbars.bookmarks.visibility";
+  let toolbar = document.querySelector("#PersonalToolbar");
+  for (let state of ["always", "never"]) {
+    info(`Testing setting toolbar state to '${state}'`);
+
+    await resetCustomization();
+    ok(CustomizableUI.inDefaultState, "Default state to begin");
+
+    setToolbarVisibility(toolbar, state, true, false);
+
+    is(
+      Services.prefs.getCharPref(prefName),
+      state,
+      "Pref updated to: " + state
+    );
+    ok(!CustomizableUI.inDefaultState, "Not in default state");
+
+    await resetCustomization();
+
+    ok(CustomizableUI.inDefaultState, "Back in default state after reset");
+    is(
+      Services.prefs.getCharPref(prefName),
+      "newtab",
+      "Pref should get reset to 'newtab'"
+    );
+  }
+});
diff --git a/browser/components/customizableui/test/browser_bookmarks_toolbar_shown_newtab.js b/browser/components/customizableui/test/browser_bookmarks_toolbar_shown_newtab.js
new file mode 100644
index 0000000000..38f385e38c
--- /dev/null
+++ b/browser/components/customizableui/test/browser_bookmarks_toolbar_shown_newtab.js
@@ -0,0 +1,34 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+"use strict";
+
+// Entering customize mode should show the toolbar as long as it's not set to "never"
+add_task(async function () {
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "Default state to begin");
+
+  let toolbar = document.querySelector("#PersonalToolbar");
+  for (let state of ["always", "never", "newtab"]) {
+    info(`Testing setting toolbar state to '${state}'`);
+
+    setToolbarVisibility(toolbar, state, true, false);
+
+    await startCustomizing();
+
+    let expected = state != "never";
+    await TestUtils.waitForCondition(
+      () => !toolbar.collapsed == expected,
+      `Waiting for toolbar visibility, state=${state}, visible=${!toolbar.collapsed}, expected=${expected}`
+    );
+    is(
+      !toolbar.collapsed,
+      expected,
+      "The toolbar should be visible when state isn't 'never'"
+    );
+
+    await endCustomizing();
+  }
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_bootstrapped_custom_toolbar.js b/browser/components/customizableui/test/browser_bootstrapped_custom_toolbar.js
new file mode 100644
index 0000000000..67325b7b36
--- /dev/null
+++ b/browser/components/customizableui/test/browser_bootstrapped_custom_toolbar.js
@@ -0,0 +1,81 @@
+/* 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";
+
+requestLongerTimeout(2);
+
+const kTestBarID = "testBar";
+const kWidgetID = "characterencoding-button";
+
+function createTestBar() {
+  let testBar = document.createXULElement("toolbar");
+  testBar.id = kTestBarID;
+  testBar.setAttribute("customizable", "true");
+  CustomizableUI.registerArea(kTestBarID, {
+    type: CustomizableUI.TYPE_TOOLBAR,
+  });
+  gNavToolbox.appendChild(testBar);
+  CustomizableUI.registerToolbarNode(testBar);
+  return testBar;
+}
+
+/**
+ * Helper function that does the following:
+ *
+ * 1) Creates a custom toolbar and registers it
+ *    with CustomizableUI.
+ * 2) Adds the widget with ID aWidgetID to that new
+ *    toolbar.
+ * 3) Enters customize mode and makes sure that the
+ *    widget is still in the right toolbar.
+ * 4) Exits customize mode, then removes and deregisters
+ *    the custom toolbar.
+ * 5) Checks that the widget has no placement.
+ * 6) Re-adds and re-registers a custom toolbar with the same
+ *    ID and options as the first one.
+ * 7) Enters customize mode and checks that the widget is
+ *    properly back in the toolbar.
+ * 8) Exits customize mode, removes and de-registers the
+ *    toolbar, and resets the toolbars to default.
+ */
+function checkRestoredPresence(aWidgetID) {
+  return (async function () {
+    let testBar = createTestBar();
+    CustomizableUI.addWidgetToArea(aWidgetID, kTestBarID);
+    let placement = CustomizableUI.getPlacementOfWidget(aWidgetID);
+    is(
+      placement.area,
+      kTestBarID,
+      "Expected " + aWidgetID + " to be in the test toolbar"
+    );
+
+    CustomizableUI.unregisterArea(testBar.id);
+    testBar.remove();
+
+    placement = CustomizableUI.getPlacementOfWidget(aWidgetID);
+    is(placement, null, "Expected " + aWidgetID + " to be in the palette");
+
+    testBar = createTestBar();
+
+    await startCustomizing();
+    placement = CustomizableUI.getPlacementOfWidget(aWidgetID);
+    is(
+      placement.area,
+      kTestBarID,
+      "Expected " + aWidgetID + " to be in the test toolbar"
+    );
+    await endCustomizing();
+
+    CustomizableUI.unregisterArea(testBar.id);
+    testBar.remove();
+
+    await resetCustomization();
+  })();
+}
+
+add_task(async function () {
+  await checkRestoredPresence("downloads-button");
+  await checkRestoredPresence("characterencoding-button");
+});
diff --git a/browser/components/customizableui/test/browser_check_tooltips_in_navbar.js b/browser/components/customizableui/test/browser_check_tooltips_in_navbar.js
new file mode 100644
index 0000000000..f36b55032d
--- /dev/null
+++ b/browser/components/customizableui/test/browser_check_tooltips_in_navbar.js
@@ -0,0 +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 check_tooltips_in_navbar() {
+  await startCustomizing();
+  let homeButtonWrapper = document.getElementById("wrapper-home-button");
+  let homeButton = document.getElementById("home-button");
+  is(
+    homeButtonWrapper.getAttribute("tooltiptext"),
+    homeButton.getAttribute("label"),
+    "the wrapper's tooltip should match the button's label"
+  );
+  ok(
+    homeButtonWrapper.getAttribute("tooltiptext"),
+    "the button should have tooltip text"
+  );
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_create_button_widget.js b/browser/components/customizableui/test/browser_create_button_widget.js
new file mode 100644
index 0000000000..ac234167cf
--- /dev/null
+++ b/browser/components/customizableui/test/browser_create_button_widget.js
@@ -0,0 +1,87 @@
+/* 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";
+
+const kButton = "test_dynamically_created_button";
+var initialLocation = gBrowser.currentURI.spec;
+
+add_task(async function () {
+  info("Check dynamically created button functionality");
+
+  // Let's create a simple button that will open about:addons.
+  let widgetSpec = {
+    id: kButton,
+    type: "button",
+    onClick() {
+      gBrowser.selectedTab = BrowserTestUtils.addTab(gBrowser, "about:addons");
+    },
+  };
+  CustomizableUI.createWidget(widgetSpec);
+  CustomizableUI.addWidgetToArea(kButton, CustomizableUI.AREA_NAVBAR);
+  ok(
+    !CustomizableUI.isWebExtensionWidget(kButton),
+    "This button should not be considered an extension widget."
+  );
+
+  // check the button's functionality in navigation bar
+  let button = document.getElementById(kButton);
+  let navBar = document.getElementById("nav-bar");
+  ok(button, "Dynamically created button exists");
+  ok(navBar.contains(button), "Dynamically created button is in the navbar");
+  await checkButtonFunctionality(button);
+
+  resetTabs();
+
+  // move the add-on button in the Panel Menu
+  CustomizableUI.addWidgetToArea(
+    kButton,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  ok(
+    !navBar.contains(button),
+    "Dynamically created button was removed from the browser bar"
+  );
+
+  await waitForOverflowButtonShown();
+
+  // check the button's functionality in the Overflow Panel.
+  await document.getElementById("nav-bar").overflowable.show();
+  var panelMenu = document.getElementById("widget-overflow-mainView");
+  let buttonInPanel = panelMenu.getElementsByAttribute("id", kButton);
+  ok(
+    panelMenu.contains(button),
+    "Dynamically created button was added to the Panel Menu"
+  );
+  await checkButtonFunctionality(buttonInPanel[0]);
+});
+
+add_task(async function asyncCleanup() {
+  resetTabs();
+
+  // reset the UI to the default state
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "The UI is in default state again.");
+
+  // destroy the widget
+  CustomizableUI.destroyWidget(kButton);
+});
+
+function resetTabs() {
+  // close all opened tabs
+  while (gBrowser.tabs.length > 1) {
+    gBrowser.removeTab(gBrowser.selectedTab);
+  }
+
+  // restore the initial tab
+  BrowserTestUtils.addTab(gBrowser, initialLocation);
+  gBrowser.removeTab(gBrowser.selectedTab);
+}
+
+async function checkButtonFunctionality(aButton) {
+  aButton.click();
+  await TestUtils.waitForCondition(
+    () => gBrowser.currentURI && gBrowser.currentURI.spec == "about:addons"
+  );
+}
diff --git a/browser/components/customizableui/test/browser_ctrl_click_panel_opening.js b/browser/components/customizableui/test/browser_ctrl_click_panel_opening.js
new file mode 100644
index 0000000000..9377c28950
--- /dev/null
+++ b/browser/components/customizableui/test/browser_ctrl_click_panel_opening.js
@@ -0,0 +1,56 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+add_task(async function test_appMenu_mainView() {
+  // On macOS, ctrl-click shouldn't open the panel because this normally opens
+  // the context menu. This happens via the `contextmenu` event which is created
+  // by widget code, so our simulated clicks do not do so, so we can't test
+  // anything on macOS:
+  if (AppConstants.platform == "macosx") {
+    ok(true, "The test is ignored on Mac");
+    return;
+  }
+
+  let mainViewID = "appMenu-protonMainView";
+  const mainView = document.getElementById(mainViewID);
+
+  let shownPromise = BrowserTestUtils.waitForEvent(mainView, "ViewShown");
+  // Should still open the panel when Ctrl key is pressed.
+  EventUtils.synthesizeMouseAtCenter(PanelUI.menuButton, { ctrlKey: true });
+  await shownPromise;
+  ok(true, "Main menu shown after button pressed");
+
+  // Close the main panel.
+  let hiddenPromise = BrowserTestUtils.waitForEvent(document, "popuphidden");
+  mainView.closest("panel").hidePopup();
+  await hiddenPromise;
+});
+
+add_task(async function test_appMenu_libraryView() {
+  // On macOS, ctrl-click shouldn't open the panel because this normally opens
+  // the context menu. This happens via the `contextmenu` event which is created
+  // by widget code, so our simulated clicks do not do so, so we can't test
+  // anything on macOS:
+  if (AppConstants.platform == "macosx") {
+    ok(true, "The test is ignored on Mac");
+    return;
+  }
+
+  CustomizableUI.addWidgetToArea("library-button", "nav-bar");
+  const button = document.getElementById("library-button");
+  await waitForElementShown(button);
+
+  // Should still open the panel when Ctrl key is pressed.
+  EventUtils.synthesizeMouseAtCenter(button, { ctrlKey: true });
+  const libraryView = document.getElementById("appMenu-libraryView");
+  let shownPromise = BrowserTestUtils.waitForEvent(libraryView, "ViewShown");
+  await shownPromise;
+  ok(true, "Library menu shown after button pressed");
+
+  // Close the Library panel.
+  let hiddenPromise = BrowserTestUtils.waitForEvent(document, "popuphidden");
+  libraryView.closest("panel").hidePopup();
+  await hiddenPromise;
+});
diff --git a/browser/components/customizableui/test/browser_currentset_post_reset.js b/browser/components/customizableui/test/browser_currentset_post_reset.js
new file mode 100644
index 0000000000..31bcd150b1
--- /dev/null
+++ b/browser/components/customizableui/test/browser_currentset_post_reset.js
@@ -0,0 +1,37 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+function checkSpacers() {
+  let navbarWidgets = CustomizableUI.getWidgetIdsInArea("nav-bar");
+  let currentSetWidgets = CustomizableUI.getTestOnlyInternalProp(
+    "CustomizableUIInternal"
+  )._getCurrentWidgetsInContainer(document.getElementById("nav-bar"));
+  navbarWidgets = navbarWidgets.filter(w => CustomizableUI.isSpecialWidget(w));
+  currentSetWidgets = currentSetWidgets.filter(w =>
+    CustomizableUI.isSpecialWidget(w)
+  );
+  Assert.deepEqual(
+    navbarWidgets,
+    currentSetWidgets,
+    "Should have the same 'special' widgets in currentset and placements"
+  );
+}
+
+/**
+ * Check that after a reset, CUI's internal bookkeeping correctly deals with flexible spacers.
+ */
+add_task(async function () {
+  await startCustomizing();
+  checkSpacers();
+
+  CustomizableUI.addWidgetToArea(
+    "spring",
+    "nav-bar",
+    4 /* Insert before the last extant spacer */
+  );
+  await gCustomizeMode.reset();
+  checkSpacers();
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_customization_context_menus.js b/browser/components/customizableui/test/browser_customization_context_menus.js
new file mode 100644
index 0000000000..0cdfb74839
--- /dev/null
+++ b/browser/components/customizableui/test/browser_customization_context_menus.js
@@ -0,0 +1,633 @@
+/* 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";
+
+requestLongerTimeout(2);
+
+const isOSX = Services.appinfo.OS === "Darwin";
+
+const overflowButton = document.getElementById("nav-bar-overflow-button");
+const overflowPanel = document.getElementById("widget-overflow");
+
+// Right-click on the stop/reload button should
+// show a context menu with options to move it.
+add_task(async function home_button_context() {
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  let stopReloadButton = document.getElementById("stop-reload-button");
+  EventUtils.synthesizeMouse(stopReloadButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToPanel", true],
+    [".customize-context-removeFromToolbar", true],
+    ["---"],
+  ];
+  if (!isOSX) {
+    expectedEntries.push(["#toggle_toolbar-menubar", true]);
+  }
+  expectedEntries.push(
+    ["#toggle_PersonalToolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", true]
+  );
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenPromise;
+});
+
+// Right-click on an empty bit of tabstrip should
+// show a context menu without options to move it,
+// but with tab-specific options instead.
+add_task(async function tabstrip_context() {
+  // ensure there are tabs to reload/bookmark:
+  let extraTab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    "http://example.com/"
+  );
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  let tabstrip = document.getElementById("tabbrowser-tabs");
+  let rect = tabstrip.getBoundingClientRect();
+  EventUtils.synthesizeMouse(tabstrip, rect.width - 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let closedTabsAvailable =
+    SessionStore.getClosedTabCountForWindow(window) == 0;
+  info("Closed tabs: " + closedTabsAvailable);
+  let expectedEntries = [
+    ["#toolbar-context-openANewTab", true],
+    ["---"],
+    ["#toolbar-context-reloadSelectedTab", true],
+    ["#toolbar-context-bookmarkSelectedTab", true],
+    ["#toolbar-context-selectAllTabs", true],
+    ["#toolbar-context-undoCloseTab", !closedTabsAvailable],
+    ["---"],
+  ];
+  if (!isOSX) {
+    expectedEntries.push(["#toggle_toolbar-menubar", true]);
+  }
+  expectedEntries.push(
+    ["#toggle_PersonalToolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", true]
+  );
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenPromise;
+  BrowserTestUtils.removeTab(extraTab);
+});
+
+// Right-click on the title bar spacer before the tabstrip should show a
+// context menu without options to move it and no tab-specific options.
+add_task(async function titlebar_spacer_context() {
+  if (!TabsInTitlebar.enabled) {
+    info("Skipping test that requires tabs in the title bar.");
+    return;
+  }
+
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  let spacer = document.querySelector(
+    "#TabsToolbar .titlebar-spacer[type='pre-tabs']"
+  );
+  EventUtils.synthesizeMouseAtCenter(spacer, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToPanel", false],
+    [".customize-context-removeFromToolbar", false],
+    ["---"],
+  ];
+  if (!isOSX) {
+    expectedEntries.push(["#toggle_toolbar-menubar", true]);
+  }
+  expectedEntries.push(
+    ["#toggle_PersonalToolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", true]
+  );
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenPromise;
+});
+
+// Right-click on an empty bit of extra toolbar should
+// show a context menu with moving options disabled,
+// and a toggle option for the extra toolbar
+add_task(async function empty_toolbar_context() {
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  let toolbar = createToolbarWithPlacements("880164_empty_toolbar", []);
+  toolbar.setAttribute("context", "toolbar-context-menu");
+  toolbar.setAttribute("toolbarname", "Fancy Toolbar for Context Menu");
+  EventUtils.synthesizeMouseAtCenter(toolbar, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToPanel", false],
+    [".customize-context-removeFromToolbar", false],
+    ["---"],
+  ];
+  if (!isOSX) {
+    expectedEntries.push(["#toggle_toolbar-menubar", true]);
+  }
+  expectedEntries.push(
+    ["#toggle_PersonalToolbar", true],
+    ["#toggle_880164_empty_toolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", true]
+  );
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenPromise;
+  removeCustomToolbars();
+});
+
+// Right-click on the urlbar-container should
+// show a context menu with disabled options to move it.
+add_task(async function urlbar_context() {
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  let urlBarContainer = document.getElementById("urlbar-container");
+  // Need to make sure not to click within an edit field.
+  EventUtils.synthesizeMouse(urlBarContainer, 100, 1, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToPanel", false],
+    [".customize-context-removeFromToolbar", false],
+    ["---"],
+  ];
+  if (!isOSX) {
+    expectedEntries.push(["#toggle_toolbar-menubar", true]);
+  }
+  expectedEntries.push(
+    ["#toggle_PersonalToolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", true]
+  );
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenPromise;
+});
+
+// Right-click on the searchbar and moving it to the menu
+// and back should move the search-container instead.
+add_task(async function searchbar_context_move_to_panel_and_back() {
+  // This is specifically testing the addToPanel function for the search bar, so
+  // we have to move it to its correct position in the navigation toolbar first.
+  // The preference will be restored when the customizations are reset later.
+  Services.prefs.setBoolPref("browser.search.widget.inNavBar", true);
+
+  let searchbar = document.getElementById("searchbar");
+  // This fails if the screen resolution is small and the search bar overflows
+  // from the nav bar.
+  await gCustomizeMode.addToPanel(searchbar);
+  let placement = CustomizableUI.getPlacementOfWidget("search-container");
+  is(
+    placement.area,
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL,
+    "Should be in panel"
+  );
+
+  await waitForOverflowButtonShown();
+
+  let shownPanelPromise = popupShown(overflowPanel);
+  overflowButton.click();
+  await shownPanelPromise;
+  let hiddenPanelPromise = popupHidden(overflowPanel);
+  overflowPanel.hidePopup();
+  await hiddenPanelPromise;
+
+  gCustomizeMode.addToToolbar(searchbar);
+  placement = CustomizableUI.getPlacementOfWidget("search-container");
+  is(placement.area, CustomizableUI.AREA_NAVBAR, "Should be in navbar");
+  await gCustomizeMode.removeFromArea(searchbar);
+  placement = CustomizableUI.getPlacementOfWidget("search-container");
+  is(placement, null, "Should be in palette");
+  CustomizableUI.reset();
+  placement = CustomizableUI.getPlacementOfWidget("search-container");
+  is(placement, null, "Should be in palette");
+});
+
+// Right-click on an item within the panel should
+// show a context menu with options to move it.
+add_task(async function context_within_panel() {
+  CustomizableUI.addWidgetToArea(
+    "new-window-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  let shownPanelPromise = popupShown(overflowPanel);
+  overflowButton.click();
+  await shownPanelPromise;
+
+  let contextMenu = document.getElementById(
+    "customizationPanelItemContextMenu"
+  );
+  let shownContextPromise = popupShown(contextMenu);
+  let newWindowButton = document.getElementById("new-window-button");
+  ok(newWindowButton, "new-window-button was found");
+  EventUtils.synthesizeMouse(newWindowButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownContextPromise;
+
+  is(overflowPanel.state, "open", "The overflow panel should still be open.");
+
+  let expectedEntries = [
+    [".customize-context-moveToToolbar", true],
+    [".customize-context-removeFromPanel", true],
+    ["---"],
+    [".viewCustomizeToolbar", true],
+  ];
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenContextPromise;
+
+  let hiddenPromise = popupHidden(overflowPanel);
+  overflowPanel.hidePopup();
+  await hiddenPromise;
+
+  CustomizableUI.removeWidgetFromArea("new-window-button");
+});
+
+// Right-click on the stop/reload button while in customization mode
+// should show a context menu with options to move it.
+add_task(async function context_home_button_in_customize_mode() {
+  await startCustomizing();
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  let stopReloadButton = document.getElementById("wrapper-stop-reload-button");
+  EventUtils.synthesizeMouse(stopReloadButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToPanel", true],
+    [".customize-context-removeFromToolbar", true],
+    ["---"],
+  ];
+  if (!isOSX) {
+    expectedEntries.push(["#toggle_toolbar-menubar", true]);
+  }
+  expectedEntries.push(
+    ["#toggle_PersonalToolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", false]
+  );
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenContextPromise;
+});
+
+// Right-click on an item in the palette should
+// show a context menu with options to move it.
+add_task(async function context_click_in_palette() {
+  let contextMenu = document.getElementById(
+    "customizationPaletteItemContextMenu"
+  );
+  let shownPromise = popupShown(contextMenu);
+  let openFileButton = document.getElementById("wrapper-open-file-button");
+  EventUtils.synthesizeMouse(openFileButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-addToToolbar", true],
+    [".customize-context-addToPanel", true],
+  ];
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenContextPromise;
+});
+
+// Right-click on an item in the panel while in customization mode
+// should show a context menu with options to move it.
+add_task(async function context_click_in_customize_mode() {
+  CustomizableUI.addWidgetToArea(
+    "new-window-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  let contextMenu = document.getElementById(
+    "customizationPanelItemContextMenu"
+  );
+  let shownPromise = popupShown(contextMenu);
+  let newWindowButton = document.getElementById("wrapper-new-window-button");
+  EventUtils.synthesizeMouse(newWindowButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToToolbar", true],
+    [".customize-context-removeFromPanel", true],
+    ["---"],
+    [".viewCustomizeToolbar", false],
+  ];
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenContextPromise;
+  CustomizableUI.removeWidgetFromArea("new-window-button");
+  await endCustomizing();
+});
+
+// Test the toolbarbutton panel context menu in customization mode
+// without opening the panel before customization mode
+add_task(async function context_click_customize_mode_panel_not_opened() {
+  CustomizableUI.addWidgetToArea(
+    "new-window-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  this.otherWin = await openAndLoadWindow(null, true);
+
+  await new Promise(resolve => waitForFocus(resolve, this.otherWin));
+
+  await startCustomizing(this.otherWin);
+
+  let contextMenu = this.otherWin.document.getElementById(
+    "customizationPanelItemContextMenu"
+  );
+  let shownPromise = popupShown(contextMenu);
+  let newWindowButton = this.otherWin.document.getElementById(
+    "wrapper-new-window-button"
+  );
+  EventUtils.synthesizeMouse(
+    newWindowButton,
+    2,
+    2,
+    { type: "contextmenu", button: 2 },
+    this.otherWin
+  );
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToToolbar", true],
+    [".customize-context-removeFromPanel", true],
+    ["---"],
+    [".viewCustomizeToolbar", false],
+  ];
+  checkContextMenu(contextMenu, expectedEntries, this.otherWin);
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenContextPromise;
+  await endCustomizing(this.otherWin);
+  CustomizableUI.removeWidgetFromArea("new-window-button");
+  await promiseWindowClosed(this.otherWin);
+  this.otherWin = null;
+
+  await new Promise(resolve => waitForFocus(resolve, window));
+});
+
+// Bug 945191 - Combined buttons show wrong context menu options
+// when they are in the toolbar.
+add_task(async function context_combined_buttons_toolbar() {
+  CustomizableUI.addWidgetToArea(
+    "zoom-controls",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  await startCustomizing();
+  let contextMenu = document.getElementById(
+    "customizationPanelItemContextMenu"
+  );
+  let shownPromise = popupShown(contextMenu);
+  let zoomControls = document.getElementById("wrapper-zoom-controls");
+  EventUtils.synthesizeMouse(zoomControls, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+  // Execute the command to move the item from the panel to the toolbar.
+  let moveToToolbar = contextMenu.querySelector(
+    ".customize-context-moveToToolbar"
+  );
+  moveToToolbar.doCommand();
+  let hiddenPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenPromise;
+  await endCustomizing();
+
+  zoomControls = document.getElementById("zoom-controls");
+  is(
+    zoomControls.parentNode.id,
+    "nav-bar-customization-target",
+    "Zoom-controls should be on the nav-bar"
+  );
+
+  contextMenu = document.getElementById("toolbar-context-menu");
+  shownPromise = popupShown(contextMenu);
+  EventUtils.synthesizeMouse(zoomControls, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    [".customize-context-moveToPanel", true],
+    [".customize-context-removeFromToolbar", true],
+    ["---"],
+  ];
+  if (!isOSX) {
+    expectedEntries.push(["#toggle_toolbar-menubar", true]);
+  }
+  expectedEntries.push(
+    ["#toggle_PersonalToolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", true]
+  );
+  checkContextMenu(contextMenu, expectedEntries);
+
+  hiddenPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenPromise;
+  await resetCustomization();
+});
+
+// Bug 947586 - After customization, panel items show wrong context menu options
+add_task(async function context_after_customization_panel() {
+  info("Check panel context menu is correct after customization");
+  await startCustomizing();
+  await endCustomizing();
+
+  CustomizableUI.addWidgetToArea(
+    "new-window-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  let shownPanelPromise = popupShown(overflowPanel);
+  overflowButton.click();
+  await shownPanelPromise;
+
+  let contextMenu = document.getElementById(
+    "customizationPanelItemContextMenu"
+  );
+  let shownContextPromise = popupShown(contextMenu);
+  let newWindowButton = document.getElementById("new-window-button");
+  ok(newWindowButton, "new-window-button was found");
+  EventUtils.synthesizeMouse(newWindowButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownContextPromise;
+
+  is(overflowPanel.state, "open", "The panel should still be open.");
+
+  let expectedEntries = [
+    [".customize-context-moveToToolbar", true],
+    [".customize-context-removeFromPanel", true],
+    ["---"],
+    [".viewCustomizeToolbar", true],
+  ];
+  checkContextMenu(contextMenu, expectedEntries);
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenContextPromise;
+
+  let hiddenPromise = popupHidden(overflowPanel);
+  overflowPanel.hidePopup();
+  await hiddenPromise;
+  CustomizableUI.removeWidgetFromArea("new-window-button");
+});
+
+// Bug 982027 - moving icon around removes custom context menu.
+add_task(async function custom_context_menus() {
+  let widgetId = "custom-context-menu-toolbarbutton";
+  let expectedContext = "myfancycontext";
+  let widget = createDummyXULButton(widgetId, "Test ctxt menu");
+  widget.setAttribute("context", expectedContext);
+  CustomizableUI.addWidgetToArea(widgetId, CustomizableUI.AREA_NAVBAR);
+  is(
+    widget.getAttribute("context"),
+    expectedContext,
+    "Should have context menu when added to the toolbar."
+  );
+
+  await startCustomizing();
+  is(
+    widget.getAttribute("context"),
+    "",
+    "Should not have own context menu in the toolbar now that we're customizing."
+  );
+  is(
+    widget.getAttribute("wrapped-context"),
+    expectedContext,
+    "Should keep own context menu wrapped when in toolbar."
+  );
+
+  let panel = document.getElementById("widget-overflow-fixed-list");
+  simulateItemDrag(widget, panel);
+  is(
+    widget.getAttribute("context"),
+    "",
+    "Should not have own context menu when in the panel."
+  );
+  is(
+    widget.getAttribute("wrapped-context"),
+    expectedContext,
+    "Should keep own context menu wrapped now that we're in the panel."
+  );
+
+  simulateItemDrag(
+    widget,
+    CustomizableUI.getCustomizationTarget(document.getElementById("nav-bar"))
+  );
+  is(
+    widget.getAttribute("context"),
+    "",
+    "Should not have own context menu when back in toolbar because we're still customizing."
+  );
+  is(
+    widget.getAttribute("wrapped-context"),
+    expectedContext,
+    "Should keep own context menu wrapped now that we're back in the toolbar."
+  );
+
+  await endCustomizing();
+  is(
+    widget.getAttribute("context"),
+    expectedContext,
+    "Should have context menu again now that we're out of customize mode."
+  );
+  CustomizableUI.removeWidgetFromArea(widgetId);
+  widget.remove();
+  ok(
+    CustomizableUI.inDefaultState,
+    "Should be in default state after removing button."
+  );
+});
+
+// Bug 1690575 - 'pin to overflow menu' and 'remove from toolbar' should be hidden
+// for flexible spaces
+add_task(async function flexible_space_context_menu() {
+  CustomizableUI.addWidgetToArea("spring", "nav-bar");
+  let springs = document.querySelectorAll("#nav-bar toolbarspring");
+  let lastSpring = springs[springs.length - 1];
+  ok(lastSpring, "we added a spring");
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  EventUtils.synthesizeMouse(lastSpring, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  let expectedEntries = [
+    ["#toggle_PersonalToolbar", true],
+    ["---"],
+    [".viewCustomizeToolbar", true],
+  ];
+
+  if (!isOSX) {
+    expectedEntries.unshift(["#toggle_toolbar-menubar", true]);
+  }
+
+  checkContextMenu(contextMenu, expectedEntries);
+  contextMenu.hidePopup();
+  gCustomizeMode.removeFromArea(lastSpring);
+  ok(!lastSpring.parentNode, "Spring should have been removed successfully.");
+});
diff --git a/browser/components/customizableui/test/browser_customizemode_contextmenu_menubuttonstate.js b/browser/components/customizableui/test/browser_customizemode_contextmenu_menubuttonstate.js
new file mode 100644
index 0000000000..78b621054c
--- /dev/null
+++ b/browser/components/customizableui/test/browser_customizemode_contextmenu_menubuttonstate.js
@@ -0,0 +1,71 @@
+"use strict";
+
+add_task(async function () {
+  ok(
+    !PanelUI.menuButton.hasAttribute("open"),
+    "Menu button should not be 'pressed' outside customize mode"
+  );
+  ok(
+    !PanelUI.menuButton.hasAttribute("disabled"),
+    "Menu button should not be disabled outside of customize mode"
+  );
+  await startCustomizing();
+
+  ok(
+    !PanelUI.menuButton.hasAttribute("open"),
+    "Menu button should still not be 'pressed' when in customize mode"
+  );
+  is(
+    PanelUI.menuButton.getAttribute("disabled"),
+    "true",
+    "Menu button should be disabled in customize mode"
+  );
+
+  let contextMenu = document.getElementById(
+    "customizationPaletteItemContextMenu"
+  );
+  let shownPromise = popupShown(contextMenu);
+  let newWindowButton = document.getElementById("wrapper-new-window-button");
+  EventUtils.synthesizeMouse(newWindowButton, 2, 2, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+  ok(
+    !PanelUI.menuButton.hasAttribute("open"),
+    "Menu button should still not be 'pressed' when in customize mode after opening a context menu"
+  );
+  is(
+    PanelUI.menuButton.getAttribute("disabled"),
+    "true",
+    "Menu button should still be disabled in customize mode"
+  );
+  is(
+    PanelUI.menuButton.getAttribute("disabled"),
+    "true",
+    "Menu button should still be disabled in customize mode after opening context menu"
+  );
+
+  let hiddenContextPromise = popupHidden(contextMenu);
+  contextMenu.hidePopup();
+  await hiddenContextPromise;
+  ok(
+    !PanelUI.menuButton.hasAttribute("open"),
+    "Menu button should still not be 'pressed' when in customize mode after hiding a context menu"
+  );
+  is(
+    PanelUI.menuButton.getAttribute("disabled"),
+    "true",
+    "Menu button should still be disabled in customize mode after hiding context menu"
+  );
+  await endCustomizing();
+
+  ok(
+    !PanelUI.menuButton.hasAttribute("open"),
+    "Menu button should not be 'pressed' after ending customize mode"
+  );
+  ok(
+    !PanelUI.menuButton.hasAttribute("disabled"),
+    "Menu button should not be disabled after ending customize mode"
+  );
+});
diff --git a/browser/components/customizableui/test/browser_customizemode_lwthemes.js b/browser/components/customizableui/test/browser_customizemode_lwthemes.js
new file mode 100644
index 0000000000..3b19566ac0
--- /dev/null
+++ b/browser/components/customizableui/test/browser_customizemode_lwthemes.js
@@ -0,0 +1,25 @@
+/* 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 startCustomizing();
+  // Find the footer buttons to test.
+  let manageLink = document.querySelector("#customization-lwtheme-link");
+
+  let waitForNewTab = BrowserTestUtils.waitForNewTab(gBrowser, "about:addons");
+  manageLink.click();
+  let addonsTab = await waitForNewTab;
+
+  is(gBrowser.currentURI.spec, "about:addons", "Manage opened about:addons");
+  BrowserTestUtils.removeTab(addonsTab);
+
+  // Wait for customize mode to be re-entered now that the customize tab is
+  // active. This is needed for endCustomizing() to work properly.
+  await TestUtils.waitForCondition(
+    () => document.documentElement.getAttribute("customizing") == "true"
+  );
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_customizemode_uidensity.js b/browser/components/customizableui/test/browser_customizemode_uidensity.js
new file mode 100644
index 0000000000..12280fc49e
--- /dev/null
+++ b/browser/components/customizableui/test/browser_customizemode_uidensity.js
@@ -0,0 +1,230 @@
+/* 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";
+
+const PREF_UI_DENSITY = "browser.uidensity";
+const PREF_AUTO_TOUCH_MODE = "browser.touchmode.auto";
+
+async function testModeMenuitem(mode, modePref) {
+  await startCustomizing();
+
+  let win = document.getElementById("main-window");
+  let popupButton = document.getElementById("customization-uidensity-button");
+  let popup = document.getElementById("customization-uidensity-menu");
+
+  // Show the popup.
+  let popupShownPromise = popupShown(popup);
+  EventUtils.synthesizeMouseAtCenter(popupButton, {});
+  await popupShownPromise;
+
+  let item = document.getElementById(
+    "customization-uidensity-menuitem-" + mode
+  );
+  let normalItem = document.getElementById(
+    "customization-uidensity-menuitem-normal"
+  );
+
+  is(
+    normalItem.getAttribute("active"),
+    "true",
+    "Normal mode menuitem should be active by default"
+  );
+
+  // Hover over the mode menuitem and wait for the event that updates the UI
+  // density.
+  let mouseoverPromise = BrowserTestUtils.waitForEvent(item, "mouseover");
+  EventUtils.synthesizeMouseAtCenter(item, { type: "mouseover" });
+  await mouseoverPromise;
+
+  is(
+    win.getAttribute("uidensity"),
+    mode,
+    `UI Density should be set to ${mode} on ${mode} menuitem hover`
+  );
+
+  is(
+    Services.prefs.getIntPref(PREF_UI_DENSITY),
+    window.gUIDensity.MODE_NORMAL,
+    `UI Density pref should still be set to normal on ${mode} menuitem hover`
+  );
+
+  // Hover the normal menuitem again and check that the UI density reset to normal.
+  EventUtils.synthesizeMouseAtCenter(normalItem, { type: "mouseover" });
+  await BrowserTestUtils.waitForCondition(() => !win.hasAttribute("uidensity"));
+
+  ok(
+    !win.hasAttribute("uidensity"),
+    `UI Density should be reset when no longer hovering the ${mode} menuitem`
+  );
+
+  // Select the custom UI density and wait for the popup to be hidden.
+  let popupHiddenPromise = popupHidden(popup);
+  EventUtils.synthesizeMouseAtCenter(item, {});
+  await popupHiddenPromise;
+
+  // Check that the click permanently changed the UI density.
+  is(
+    win.getAttribute("uidensity"),
+    mode,
+    `UI Density should be set to ${mode} on ${mode} menuitem click`
+  );
+  is(
+    Services.prefs.getIntPref(PREF_UI_DENSITY),
+    modePref,
+    `UI Density pref should be set to ${mode} when clicking the ${mode} menuitem`
+  );
+
+  // Open the popup again.
+  popupShownPromise = popupShown(popup);
+  EventUtils.synthesizeMouseAtCenter(popupButton, {});
+  await popupShownPromise;
+
+  // Check that the menuitem is still active after opening and closing the popup.
+  is(
+    item.getAttribute("active"),
+    "true",
+    `${mode} mode menuitem should be active`
+  );
+
+  // Hide the popup again.
+  popupHiddenPromise = popupHidden(popup);
+  EventUtils.synthesizeMouseAtCenter(popupButton, {});
+  await popupHiddenPromise;
+
+  // Check that the menuitem is still active after re-opening customize mode.
+  await endCustomizing();
+  await startCustomizing();
+
+  popupShownPromise = popupShown(popup);
+  EventUtils.synthesizeMouseAtCenter(popupButton, {});
+  await popupShownPromise;
+
+  is(
+    item.getAttribute("active"),
+    "true",
+    `${mode} mode menuitem should be active after entering and exiting customize mode`
+  );
+
+  // Click the normal menuitem and check that the density is reset.
+  popupHiddenPromise = popupHidden(popup);
+  EventUtils.synthesizeMouseAtCenter(normalItem, {});
+  await popupHiddenPromise;
+
+  ok(
+    !win.hasAttribute("uidensity"),
+    "UI Density should be reset when clicking the normal menuitem"
+  );
+
+  is(
+    Services.prefs.getIntPref(PREF_UI_DENSITY),
+    window.gUIDensity.MODE_NORMAL,
+    "UI Density pref should be set to normal."
+  );
+
+  // Show the popup and click on the mode menuitem again to test the
+  // reset default feature.
+  popupShownPromise = popupShown(popup);
+  EventUtils.synthesizeMouseAtCenter(popupButton, {});
+  await popupShownPromise;
+
+  popupHiddenPromise = popupHidden(popup);
+  EventUtils.synthesizeMouseAtCenter(item, {});
+  await popupHiddenPromise;
+
+  is(
+    win.getAttribute("uidensity"),
+    mode,
+    `UI Density should be set to ${mode} on ${mode} menuitem click`
+  );
+
+  is(
+    Services.prefs.getIntPref(PREF_UI_DENSITY),
+    modePref,
+    `UI Density pref should be set to ${mode} when clicking the ${mode} menuitem`
+  );
+
+  await gCustomizeMode.reset();
+
+  ok(
+    !win.hasAttribute("uidensity"),
+    "UI Density should be reset when clicking the normal menuitem"
+  );
+
+  is(
+    Services.prefs.getIntPref(PREF_UI_DENSITY),
+    window.gUIDensity.MODE_NORMAL,
+    "UI Density pref should be set to normal."
+  );
+
+  await endCustomizing();
+}
+
+add_task(async function test_touch_mode_menuitem() {
+  // OSX doesn't get touch mode for now.
+  if (AppConstants.platform == "macosx") {
+    is(
+      document.getElementById("customization-uidensity-menuitem-touch"),
+      null,
+      "There's no touch option on Mac OSX"
+    );
+    return;
+  }
+
+  await testModeMenuitem("touch", window.gUIDensity.MODE_TOUCH);
+
+  // Test the checkbox for automatic Touch Mode transition
+  // in Windows 10 Tablet Mode.
+  if (AppConstants.isPlatformAndVersionAtLeast("win", "10")) {
+    await startCustomizing();
+
+    let popupButton = document.getElementById("customization-uidensity-button");
+    let popup = document.getElementById("customization-uidensity-menu");
+    let popupShownPromise = popupShown(popup);
+    EventUtils.synthesizeMouseAtCenter(popupButton, {});
+    await popupShownPromise;
+
+    let checkbox = document.getElementById(
+      "customization-uidensity-autotouchmode-checkbox"
+    );
+    ok(checkbox.checked, "Checkbox should be checked by default");
+
+    // Test toggling the checkbox.
+    EventUtils.synthesizeMouseAtCenter(checkbox, {});
+    is(
+      Services.prefs.getBoolPref(PREF_AUTO_TOUCH_MODE),
+      false,
+      "Automatic Touch Mode is off when the checkbox is unchecked."
+    );
+
+    EventUtils.synthesizeMouseAtCenter(checkbox, {});
+    is(
+      Services.prefs.getBoolPref(PREF_AUTO_TOUCH_MODE),
+      true,
+      "Automatic Touch Mode is on when the checkbox is checked."
+    );
+
+    // Test reset to defaults.
+    EventUtils.synthesizeMouseAtCenter(checkbox, {});
+    is(
+      Services.prefs.getBoolPref(PREF_AUTO_TOUCH_MODE),
+      false,
+      "Automatic Touch Mode is off when the checkbox is unchecked."
+    );
+
+    await gCustomizeMode.reset();
+    is(
+      Services.prefs.getBoolPref(PREF_AUTO_TOUCH_MODE),
+      true,
+      "Automatic Touch Mode is on when the checkbox is checked."
+    );
+  }
+});
+
+add_task(async function cleanup() {
+  await endCustomizing();
+
+  Services.prefs.clearUserPref(PREF_UI_DENSITY);
+  Services.prefs.clearUserPref(PREF_AUTO_TOUCH_MODE);
+});
diff --git a/browser/components/customizableui/test/browser_disable_commands_customize.js b/browser/components/customizableui/test/browser_disable_commands_customize.js
new file mode 100644
index 0000000000..f3eb06efbe
--- /dev/null
+++ b/browser/components/customizableui/test/browser_disable_commands_customize.js
@@ -0,0 +1,86 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Most commands don't make sense in customize mode. Check that they're
+ * disabled, so shortcuts can't activate them either. Also check that
+ * some basic commands (close tab/window, quit, new tab, new window)
+ * remain functional.
+ */
+add_task(async function test_disable_commands() {
+  let disabledCommands = ["cmd_print", "Browser:SavePage", "Browser:SendLink"];
+  let enabledCommands = [
+    "cmd_newNavigatorTab",
+    "cmd_newNavigator",
+    "cmd_quitApplication",
+    "cmd_close",
+    "cmd_closeWindow",
+  ];
+
+  function checkDisabled() {
+    for (let cmd of disabledCommands) {
+      is(
+        document.getElementById(cmd).getAttribute("disabled"),
+        "true",
+        `Command ${cmd} should be disabled`
+      );
+    }
+    for (let cmd of enabledCommands) {
+      ok(
+        !document.getElementById(cmd).hasAttribute("disabled"),
+        `Command ${cmd} should NOT be disabled`
+      );
+    }
+  }
+  await startCustomizing();
+
+  checkDisabled();
+
+  // Do a reset just for fun, making sure we don't accidentally
+  // break things:
+  await gCustomizeMode.reset();
+
+  checkDisabled();
+
+  await endCustomizing();
+  for (let cmd of disabledCommands.concat(enabledCommands)) {
+    ok(
+      !document.getElementById(cmd).hasAttribute("disabled"),
+      `Command ${cmd} should NOT be disabled after customize mode`
+    );
+  }
+});
+
+/**
+ * When buttons are connected to a command, they should not get
+ * disabled just because we move them.
+ */
+add_task(async function test_dont_disable_when_moving() {
+  let button = gNavToolbox.palette.querySelector("#print-button");
+  ok(button.hasAttribute("command"), "Button should have a command attribute.");
+  await startCustomizing();
+  CustomizableUI.addWidgetToArea("print-button", "nav-bar");
+  await endCustomizing();
+  ok(
+    !button.hasAttribute("disabled"),
+    "Should not have disabled attribute after adding the button."
+  );
+  ok(
+    button.hasAttribute("command"),
+    "Button should still have a command attribute."
+  );
+
+  await startCustomizing();
+  await gCustomizeMode.reset();
+  await endCustomizing();
+  ok(
+    !button.hasAttribute("disabled"),
+    "Should not have disabled attribute when resetting in customize mode"
+  );
+  ok(
+    button.hasAttribute("command"),
+    "Button should still have a command attribute."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_drag_outside_palette.js b/browser/components/customizableui/test/browser_drag_outside_palette.js
new file mode 100644
index 0000000000..2785a08896
--- /dev/null
+++ b/browser/components/customizableui/test/browser_drag_outside_palette.js
@@ -0,0 +1,53 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check that moving items from the toolbar or panel to the palette by
+ * dropping on the panel container (not inside the visible panel) works.
+ */
+add_task(async function () {
+  await startCustomizing();
+  let panelContainer = document.getElementById("customization-panel-container");
+  // Try dragging an item from the navbar:
+  let stopReloadButton = document.getElementById("stop-reload-button");
+  let oldNavbarPlacements = CustomizableUI.getWidgetIdsInArea("nav-bar");
+  simulateItemDrag(stopReloadButton, panelContainer);
+  assertAreaPlacements(
+    CustomizableUI.AREA_NAVBAR,
+    oldNavbarPlacements.filter(w => w != "stop-reload-button")
+  );
+  ok(
+    stopReloadButton.closest("#customization-palette"),
+    "Button should be in the palette"
+  );
+
+  // Put it in the panel and try again from there:
+  let panelHolder = document.getElementById("customization-panelHolder");
+  simulateItemDrag(stopReloadButton, panelHolder);
+  assertAreaPlacements(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL, [
+    "stop-reload-button",
+  ]);
+
+  simulateItemDrag(stopReloadButton, panelContainer);
+  assertAreaPlacements(CustomizableUI.AREA_FIXED_OVERFLOW_PANEL, []);
+
+  ok(
+    stopReloadButton.closest("#customization-palette"),
+    "Button should be in the palette"
+  );
+
+  // Check we can't move non-removable items like this:
+  let urlbar = document.getElementById("urlbar-container");
+  simulateItemDrag(urlbar, panelContainer);
+  assertAreaPlacements(
+    CustomizableUI.AREA_NAVBAR,
+    oldNavbarPlacements.filter(w => w != "stop-reload-button")
+  );
+});
+
+registerCleanupFunction(async function () {
+  await gCustomizeMode.reset();
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_editcontrols_update.js b/browser/components/customizableui/test/browser_editcontrols_update.js
new file mode 100644
index 0000000000..9f064e521a
--- /dev/null
+++ b/browser/components/customizableui/test/browser_editcontrols_update.js
@@ -0,0 +1,307 @@
+// This test checks that the edit command enabled state (cut/paste) is updated
+// properly when the edit controls are on the toolbar, popup and not present.
+// It also verifies that the performance optimiation implemented by
+// updateEditUIVisibility in browser.js is applied.
+
+let isMac = navigator.platform.indexOf("Mac") == 0;
+
+function checkState(allowCut, desc, testWindow = window) {
+  is(
+    testWindow.document.getElementById("cmd_cut").getAttribute("disabled") ==
+      "true",
+    !allowCut,
+    desc + " - cut"
+  );
+  is(
+    testWindow.document.getElementById("cmd_paste").getAttribute("disabled") ==
+      "true",
+    false,
+    desc + " - paste"
+  );
+}
+
+// Add a special controller to the urlbar and browser to listen in on when
+// commands are being updated. Return a promise that resolves when 'count'
+// updates have occurred.
+function expectCommandUpdate(count, testWindow = window) {
+  return new Promise((resolve, reject) => {
+    let overrideController = {
+      supportsCommand(cmd) {
+        return cmd == "cmd_delete";
+      },
+      isCommandEnabled(cmd) {
+        if (!count) {
+          ok(false, "unexpected update");
+          reject();
+        }
+
+        if (!--count) {
+          testWindow.gURLBar.inputField.controllers.removeControllerAt(
+            0,
+            overrideController
+          );
+          testWindow.gBrowser.selectedBrowser.controllers.removeControllerAt(
+            0,
+            overrideController
+          );
+          resolve(true);
+        }
+      },
+    };
+
+    if (!count) {
+      SimpleTest.executeSoon(() => {
+        testWindow.gURLBar.inputField.controllers.removeControllerAt(
+          0,
+          overrideController
+        );
+        testWindow.gBrowser.selectedBrowser.controllers.removeControllerAt(
+          0,
+          overrideController
+        );
+        resolve(false);
+      });
+    }
+
+    testWindow.gURLBar.inputField.controllers.insertControllerAt(
+      0,
+      overrideController
+    );
+    testWindow.gBrowser.selectedBrowser.controllers.insertControllerAt(
+      0,
+      overrideController
+    );
+  });
+}
+
+// Call this between `.select()` to make sure the selection actually changes
+// and thus TextInputListener::UpdateTextInputCommands() is called.
+function deselectURLBarAndSpin() {
+  gURLBar.inputField.setSelectionRange(0, 0);
+  return new Promise(setTimeout);
+}
+
+add_task(async function test_init() {
+  // Put something on the clipboard to verify that the paste button is properly enabled during the test.
+  let clipboardHelper = Cc["@mozilla.org/widget/clipboardhelper;1"].getService(
+    Ci.nsIClipboardHelper
+  );
+  await new Promise(resolve => {
+    SimpleTest.waitForClipboard(
+      "Sample",
+      function () {
+        clipboardHelper.copyString("Sample");
+      },
+      resolve
+    );
+  });
+
+  // Open and close the panel first so that it is fully initialized.
+  await gCUITestUtils.openMainMenu();
+  await gCUITestUtils.hideMainMenu();
+});
+
+// Test updating when the panel is open with the edit-controls on the panel.
+// Updates should occur.
+add_task(async function test_panelui_opened() {
+  document.commandDispatcher.unlock();
+  gURLBar.focus();
+  gURLBar.value = "test";
+
+  await gCUITestUtils.openMainMenu();
+
+  checkState(false, "Update when edit-controls is on panel and visible");
+
+  let overridePromise = expectCommandUpdate(1);
+  gURLBar.select();
+  await overridePromise;
+
+  checkState(
+    true,
+    "Update when edit-controls is on panel and selection changed"
+  );
+
+  overridePromise = expectCommandUpdate(0);
+  await gCUITestUtils.hideMainMenu();
+  await overridePromise;
+
+  // Check that updates do not occur after the panel has been closed.
+  checkState(true, "Update when edit-controls is on panel and hidden");
+
+  // Mac will update the enabled state even when the panel is closed so that
+  // main menubar shortcuts will work properly.
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(isMac ? 1 : 0);
+  gURLBar.select();
+  await overridePromise;
+  checkState(
+    true,
+    "Update when edit-controls is on panel, hidden and selection changed"
+  );
+});
+
+// Test updating when the edit-controls are moved to the toolbar.
+add_task(async function test_panelui_customize_to_toolbar() {
+  await startCustomizing();
+  let navbar = document.getElementById("nav-bar");
+  simulateItemDrag(
+    document.getElementById("edit-controls"),
+    CustomizableUI.getCustomizationTarget(navbar),
+    "end"
+  );
+  await endCustomizing();
+
+  // updateEditUIVisibility should be called when customization ends but isn't. See bug 1359790.
+  updateEditUIVisibility();
+
+  // The URL bar may have been focused to begin with, which means
+  // that subsequent calls to focus it won't result in command
+  // updates, so we'll make sure to blur it.
+  gURLBar.blur();
+
+  let overridePromise = expectCommandUpdate(1);
+  gURLBar.select();
+  gURLBar.focus();
+  gURLBar.value = "other";
+  await overridePromise;
+  checkState(false, "Update when edit-controls on toolbar and focused");
+
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(1);
+  gURLBar.select();
+  await overridePromise;
+  checkState(
+    true,
+    "Update when edit-controls on toolbar and selection changed"
+  );
+
+  const kOverflowPanel = document.getElementById("widget-overflow");
+
+  let originalWidth = window.outerWidth;
+  registerCleanupFunction(async function () {
+    kOverflowPanel.removeAttribute("animate");
+    window.resizeTo(originalWidth, window.outerHeight);
+    await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+    CustomizableUI.reset();
+  });
+
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () =>
+      navbar.hasAttribute("overflowing") &&
+      !navbar.querySelector("edit-controls")
+  );
+
+  // Mac will update the enabled state even when the buttons are overflowing,
+  // so main menubar shortcuts will work properly.
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(isMac ? 1 : 0);
+  gURLBar.select();
+  await overridePromise;
+  checkState(
+    true,
+    "Update when edit-controls is on overflow panel, hidden and selection changed"
+  );
+
+  // Check that we get an update if we select content while the panel is open.
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(1);
+  await navbar.overflowable.show();
+  gURLBar.select();
+  await overridePromise;
+
+  // And that we don't (except on mac) when the panel is hidden.
+  kOverflowPanel.hidePopup();
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(isMac ? 1 : 0);
+  gURLBar.select();
+  await overridePromise;
+
+  window.resizeTo(originalWidth, window.outerHeight);
+  await TestUtils.waitForCondition(() => !navbar.hasAttribute("overflowing"));
+
+  CustomizableUI.addWidgetToArea(
+    "edit-controls",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  // updateEditUIVisibility should be called when customization happens but isn't. See bug 1359790.
+  updateEditUIVisibility();
+
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(isMac ? 1 : 0);
+  gURLBar.select();
+  await overridePromise;
+
+  // Check that we get an update if we select content while the panel is open.
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(1);
+  await navbar.overflowable.show();
+  gURLBar.select();
+  await overridePromise;
+
+  // And that we don't (except on mac) when the panel is hidden.
+  kOverflowPanel.hidePopup();
+  await deselectURLBarAndSpin();
+  overridePromise = expectCommandUpdate(isMac ? 1 : 0);
+  gURLBar.select();
+  await overridePromise;
+});
+
+// Test updating when the edit-controls are moved to the palette.
+add_task(async function test_panelui_customize_to_palette() {
+  await startCustomizing();
+  let palette = document.getElementById("customization-palette");
+  simulateItemDrag(document.getElementById("edit-controls"), palette);
+  await endCustomizing();
+
+  // updateEditUIVisibility should be called when customization ends but isn't. See bug 1359790.
+  updateEditUIVisibility();
+
+  let overridePromise = expectCommandUpdate(isMac ? 1 : 0);
+  gURLBar.focus();
+  gURLBar.value = "other";
+  gURLBar.select();
+  await overridePromise;
+
+  // If the UI isn't found, the command is set to be enabled.
+  checkState(
+    true,
+    "Update when edit-controls is on palette, hidden and selection changed"
+  );
+});
+
+add_task(async function finish() {
+  await resetCustomization();
+});
+
+// Test updating in the initial state when the edit-controls are on the panel but
+// have not yet been created. This needs to be done in a new window to ensure that
+// other tests haven't opened the panel.
+add_task(async function test_initial_state() {
+  let testWindow = await BrowserTestUtils.openNewBrowserWindow();
+  await SimpleTest.promiseFocus(testWindow);
+
+  // For focusing the URL bar to have an effect, we need to ensure the URL bar isn't
+  // initially focused:
+  testWindow.gBrowser.selectedTab.focus();
+  await TestUtils.waitForCondition(() => !testWindow.gURLBar.focused);
+
+  let overridePromise = expectCommandUpdate(isMac, testWindow);
+
+  testWindow.gURLBar.focus();
+  testWindow.gURLBar.value = "test";
+
+  await overridePromise;
+
+  // Commands won't update when no edit UI is present. They default to being
+  // enabled so that keyboard shortcuts will work. The real enabled state will
+  // be checked when shortcut is pressed.
+  checkState(
+    !isMac,
+    "No update when edit-controls is on panel and not visible",
+    testWindow
+  );
+
+  await BrowserTestUtils.closeWindow(testWindow);
+  await SimpleTest.promiseFocus(window);
+});
diff --git a/browser/components/customizableui/test/browser_exit_background_customize_mode.js b/browser/components/customizableui/test/browser_exit_background_customize_mode.js
new file mode 100644
index 0000000000..b89efef582
--- /dev/null
+++ b/browser/components/customizableui/test/browser_exit_background_customize_mode.js
@@ -0,0 +1,44 @@
+"use strict";
+
+/**
+ * Tests that if customize mode is currently attached to a background
+ * tab, and that tab browses to a new location, that customize mode
+ * is detached from that tab.
+ */
+add_task(async function test_exit_background_customize_mode() {
+  let nonCustomizingTab = gBrowser.selectedTab;
+
+  Assert.equal(
+    gBrowser.tabContainer.querySelector("tab[customizemode=true]"),
+    null,
+    "Should not have a tab marked as being the customize tab now."
+  );
+
+  await startCustomizing();
+  is(gBrowser.tabs.length, 2, "Should have 2 tabs");
+
+  let custTab = gBrowser.selectedTab;
+
+  let finishedCustomizing = BrowserTestUtils.waitForEvent(
+    gNavToolbox,
+    "aftercustomization"
+  );
+  await BrowserTestUtils.switchTab(gBrowser, nonCustomizingTab);
+  await finishedCustomizing;
+
+  let newURL = "http://example.com/";
+  BrowserTestUtils.loadURIString(custTab.linkedBrowser, newURL);
+  await BrowserTestUtils.browserLoaded(custTab.linkedBrowser, false, newURL);
+
+  Assert.equal(
+    gBrowser.tabContainer.querySelector("tab[customizemode=true]"),
+    null,
+    "Should not have a tab marked as being the customize tab now."
+  );
+
+  await startCustomizing();
+  is(gBrowser.tabs.length, 3, "Should have 3 tabs now");
+
+  await endCustomizing();
+  BrowserTestUtils.removeTab(custTab);
+});
diff --git a/browser/components/customizableui/test/browser_flexible_space_area.js b/browser/components/customizableui/test/browser_flexible_space_area.js
new file mode 100644
index 0000000000..f3189096de
--- /dev/null
+++ b/browser/components/customizableui/test/browser_flexible_space_area.js
@@ -0,0 +1,48 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+function getSpringCount(area) {
+  return CustomizableUI.getWidgetIdsInArea(area).filter(id =>
+    id.includes("spring")
+  ).length;
+}
+
+/**
+ * Check that no matter where we add a flexible space, we
+ * never end up without a flexible space in the palette.
+ */
+add_task(async function test_flexible_space_addition() {
+  await startCustomizing();
+  let palette = document.getElementById("customization-palette");
+  // Make the bookmarks toolbar visible:
+  CustomizableUI.setToolbarVisibility(CustomizableUI.AREA_BOOKMARKS, true);
+  let areas = [CustomizableUI.AREA_NAVBAR, CustomizableUI.AREA_BOOKMARKS];
+  if (AppConstants.platform != "macosx") {
+    areas.push(CustomizableUI.AREA_MENUBAR);
+  }
+
+  for (let area of areas) {
+    let spacer = palette.querySelector("toolbarspring");
+    let toolbar = document.getElementById(area);
+    toolbar = CustomizableUI.getCustomizationTarget(toolbar);
+
+    let springCount = getSpringCount(area);
+    simulateItemDrag(spacer, toolbar);
+    // Check we added the spring:
+    is(
+      springCount + 1,
+      getSpringCount(area),
+      "Should now have an extra spring"
+    );
+
+    // Check there's still one in the palette:
+    let newSpacer = palette.querySelector("toolbarspring");
+    ok(newSpacer, "Should have created a new spring");
+  }
+});
+registerCleanupFunction(async function asyncCleanup() {
+  await endCustomizing();
+  await resetCustomization();
+});
diff --git a/browser/components/customizableui/test/browser_help_panel_cloning.js b/browser/components/customizableui/test/browser_help_panel_cloning.js
new file mode 100644
index 0000000000..4234a52cd8
--- /dev/null
+++ b/browser/components/customizableui/test/browser_help_panel_cloning.js
@@ -0,0 +1,90 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/* global PanelUI */
+
+let gAppMenuStrings = new Localization(
+  ["branding/brand.ftl", "browser/appmenu.ftl"],
+  true
+);
+
+const CLONED_ATTRS = ["command", "oncommand", "onclick", "key", "disabled"];
+
+/**
+ * Tests that the Help panel inside of the AppMenu properly clones
+ * the items from the Help menupopup. Also ensures that the AppMenu
+ * string variants for those menuitems exist inside of appmenu.ftl.
+ */
+add_task(async function test_help_panel_cloning() {
+  await gCUITestUtils.openMainMenu();
+  registerCleanupFunction(async () => {
+    await gCUITestUtils.hideMainMenu();
+  });
+
+  // Showing the Help panel should be enough to get the menupopup to
+  // populate itself.
+  let anchor = document.getElementById("PanelUI-menu-button");
+  PanelUI.showHelpView(anchor);
+
+  let appMenuHelpSubview = document.getElementById("PanelUI-helpView");
+  await BrowserTestUtils.waitForEvent(appMenuHelpSubview, "ViewShowing");
+
+  let helpMenuPopup = document.getElementById("menu_HelpPopup");
+  let helpMenuPopupItems = helpMenuPopup.querySelectorAll("menuitem");
+
+  for (let helpMenuPopupItem of helpMenuPopupItems) {
+    if (helpMenuPopupItem.hidden) {
+      continue;
+    }
+
+    let appMenuHelpId = "appMenu_" + helpMenuPopupItem.id;
+    info(`Checking ${appMenuHelpId}`);
+
+    let appMenuHelpItem = appMenuHelpSubview.querySelector(`#${appMenuHelpId}`);
+    Assert.ok(appMenuHelpItem, "Should have found a cloned AppMenu help item");
+
+    let appMenuHelpItemL10nId = appMenuHelpItem.dataset.l10nId;
+    // There is a convention that the Help menu item should have an
+    // appmenu-data-l10n-id attribute set as the AppMenu-specific localization
+    // id.
+    Assert.equal(
+      helpMenuPopupItem.getAttribute("appmenu-data-l10n-id"),
+      appMenuHelpItemL10nId,
+      "Help menuitem supplied a data-l10n-id for the AppMenu Help item"
+    );
+
+    let [strings] = gAppMenuStrings.formatMessagesSync([
+      { id: appMenuHelpItemL10nId },
+    ]);
+    Assert.ok(strings, "Should have found strings for the AppMenu help item");
+
+    // Make sure the CLONED_ATTRs are actually cloned.
+    for (let attr of CLONED_ATTRS) {
+      if (attr == "oncommand" && helpMenuPopupItem.hasAttribute("command")) {
+        // If the original element had a "command" attribute set, then the
+        // cloned element will have its "oncommand" attribute set to equal
+        // the "oncommand" attribute of the <command> pointed to via the
+        // original's "command" attribute once it is inserted into the DOM.
+        //
+        // This is by virtue of the broadcasting ability of XUL <command>
+        // elements.
+        let commandNode = document.getElementById(
+          helpMenuPopupItem.getAttribute("command")
+        );
+        Assert.equal(
+          commandNode.getAttribute("oncommand"),
+          appMenuHelpItem.getAttribute("oncommand"),
+          "oncommand was properly cloned."
+        );
+      } else {
+        Assert.equal(
+          helpMenuPopupItem.getAttribute(attr),
+          appMenuHelpItem.getAttribute(attr),
+          `${attr} attribute was cloned.`
+        );
+      }
+    }
+  }
+});
diff --git a/browser/components/customizableui/test/browser_hidden_widget_overflow.js b/browser/components/customizableui/test/browser_hidden_widget_overflow.js
new file mode 100644
index 0000000000..0221bfc336
--- /dev/null
+++ b/browser/components/customizableui/test/browser_hidden_widget_overflow.js
@@ -0,0 +1,115 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Tests that if only hidden widgets are overflowed that the
+ * OverflowableToolbar won't show the overflow panel anchor.
+ */
+
+const kHiddenButtonID = "fake-hidden-button";
+const kDisplayNoneButtonID = "display-none-button";
+const kWebExtensionButtonID1 = "fake-webextension-button-1";
+const kWebExtensionButtonID2 = "fake-webextension-button-2";
+let gWin = null;
+
+add_setup(async function () {
+  gWin = await BrowserTestUtils.openNewBrowserWindow();
+
+  // To make it easier to write a test where we can control overflowing
+  // for a test that can run in a bunch of environments with slightly
+  // different rules on when things will overflow, we'll go ahead and
+  // just remove everything removable from the nav-bar by default. Then
+  // we'll add our hidden item, and a single WebExtension item, and
+  // force toolbar overflow.
+  let widgetIDs = CustomizableUI.getWidgetIdsInArea(CustomizableUI.AREA_NAVBAR);
+  for (let widgetID of widgetIDs) {
+    if (CustomizableUI.isWidgetRemovable(widgetID)) {
+      CustomizableUI.removeWidgetFromArea(widgetID);
+    }
+  }
+
+  CustomizableUI.createWidget({
+    id: kWebExtensionButtonID1,
+    label: "Test WebExtension widget 1",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+    webExtension: true,
+  });
+
+  CustomizableUI.createWidget({
+    id: kWebExtensionButtonID2,
+    label: "Test WebExtension widget 2",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+    webExtension: true,
+  });
+
+  // Let's force the WebExtension widgets to be significantly wider. This
+  // just makes it easier to ensure that both of these (which are to the left
+  // of the hidden widget) get overflowed.
+  for (let webExtID of [kWebExtensionButtonID1, kWebExtensionButtonID2]) {
+    let webExtNode = CustomizableUI.getWidget(webExtID).forWindow(gWin).node;
+    webExtNode.style.minWidth = "100px";
+  }
+
+  CustomizableUI.createWidget({
+    id: kHiddenButtonID,
+    label: "Test hidden=true widget",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  });
+
+  // Now hide the button with hidden=true so that it has no dimensions.
+  let hiddenButtonNode =
+    CustomizableUI.getWidget(kHiddenButtonID).forWindow(gWin).node;
+  hiddenButtonNode.hidden = true;
+
+  CustomizableUI.createWidget({
+    id: kDisplayNoneButtonID,
+    label: "Test display:none widget",
+    defaultArea: CustomizableUI.AREA_NAVBAR,
+  });
+
+  // Now hide the button with display: none so that it has no dimensions.
+  let displayNoneButtonNode =
+    CustomizableUI.getWidget(kDisplayNoneButtonID).forWindow(gWin).node;
+  displayNoneButtonNode.style.display = "none";
+
+  registerCleanupFunction(async () => {
+    CustomizableUI.destroyWidget(kWebExtensionButtonID1);
+    CustomizableUI.destroyWidget(kWebExtensionButtonID2);
+    CustomizableUI.destroyWidget(kHiddenButtonID);
+    CustomizableUI.destroyWidget(kDisplayNoneButtonID);
+    await BrowserTestUtils.closeWindow(gWin);
+    await CustomizableUI.reset();
+  });
+});
+
+add_task(async function test_hidden_widget_overflow() {
+  gWin.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+
+  // Wait until the left-most fake WebExtension button is overflowing.
+  let webExtNode = CustomizableUI.getWidget(kWebExtensionButtonID1).forWindow(
+    gWin
+  ).node;
+  await BrowserTestUtils.waitForMutationCondition(
+    webExtNode,
+    { attributes: true },
+    () => {
+      return webExtNode.hasAttribute("overflowedItem");
+    }
+  );
+
+  let hiddenButtonNode =
+    CustomizableUI.getWidget(kHiddenButtonID).forWindow(gWin).node;
+  Assert.ok(
+    hiddenButtonNode.hasAttribute("overflowedItem"),
+    "Hidden button should be overflowed."
+  );
+
+  let overflowButton = gWin.document.getElementById("nav-bar-overflow-button");
+
+  Assert.ok(
+    !BrowserTestUtils.is_visible(overflowButton),
+    "Overflow panel button should be hidden."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_history_after_appMenu.js b/browser/components/customizableui/test/browser_history_after_appMenu.js
new file mode 100644
index 0000000000..89c4b467a2
--- /dev/null
+++ b/browser/components/customizableui/test/browser_history_after_appMenu.js
@@ -0,0 +1,35 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Checks that opening the History view using the default toolbar button works
+ * also while the view is displayed in the main menu.
+ */
+add_task(async function test_history_after_appMenu() {
+  // First add the button to the toolbar and wait for it to show up:
+  CustomizableUI.addWidgetToArea("history-panelmenu", "nav-bar");
+  registerCleanupFunction(() =>
+    CustomizableUI.removeWidgetFromArea("history-panelmenu")
+  );
+  await waitForElementShown(document.getElementById("history-panelmenu"));
+
+  let historyView = PanelMultiView.getViewNode(document, "PanelUI-history");
+  // Open the main menu.
+  await gCUITestUtils.openMainMenu();
+
+  // Show the History view as a subview of the main menu.
+  document.getElementById("appMenu-history-button").click();
+  await BrowserTestUtils.waitForEvent(historyView, "ViewShown");
+
+  // Show the History view as the main view of the History panel.
+  document.getElementById("history-panelmenu").click();
+  await BrowserTestUtils.waitForEvent(historyView, "ViewShown");
+
+  // Close the history panel.
+  let historyPanel = historyView.closest("panel");
+  let promise = BrowserTestUtils.waitForEvent(historyPanel, "popuphidden");
+  historyPanel.hidePopup();
+  await promise;
+});
diff --git a/browser/components/customizableui/test/browser_history_recently_closed.js b/browser/components/customizableui/test/browser_history_recently_closed.js
new file mode 100644
index 0000000000..01039ed960
--- /dev/null
+++ b/browser/components/customizableui/test/browser_history_recently_closed.js
@@ -0,0 +1,198 @@
+/* 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";
+
+const TEST_PATH = getRootDirectory(gTestPath).replace(
+  "chrome://mochitests/content",
+  "http://example.com"
+);
+
+add_task(async function testRecentlyClosedDisabled() {
+  info("Check history recently closed tabs/windows section");
+
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  // We need to make sure the history is cleared before starting the test
+  await Sanitizer.sanitize(["history"]);
+
+  await openHistoryPanel();
+
+  let recentlyClosedTabs = document.getElementById("appMenuRecentlyClosedTabs");
+  let recentlyClosedWindows = document.getElementById(
+    "appMenuRecentlyClosedWindows"
+  );
+
+  // Wait for the disabled attribute to change, as we receive
+  // the "viewshown" event before this changes
+  await BrowserTestUtils.waitForCondition(
+    () => recentlyClosedTabs.getAttribute("disabled"),
+    "Waiting for button to become disabled"
+  );
+  Assert.ok(
+    recentlyClosedTabs.getAttribute("disabled"),
+    "Recently closed tabs button disabled"
+  );
+  Assert.ok(
+    recentlyClosedWindows.getAttribute("disabled"),
+    "Recently closed windows button disabled"
+  );
+
+  await hideHistoryPanel();
+
+  gBrowser.selectedTab.focus();
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    gBrowser,
+    TEST_PATH + "dummy_history_item.html"
+  );
+  BrowserTestUtils.removeTab(tab);
+
+  await openHistoryPanel();
+
+  await BrowserTestUtils.waitForCondition(
+    () => !recentlyClosedTabs.getAttribute("disabled"),
+    "Waiting for button to be enabled"
+  );
+  Assert.ok(
+    !recentlyClosedTabs.getAttribute("disabled"),
+    "Recently closed tabs is available"
+  );
+  Assert.ok(
+    recentlyClosedWindows.getAttribute("disabled"),
+    "Recently closed windows button disabled"
+  );
+
+  await hideHistoryPanel();
+
+  let newWin = await BrowserTestUtils.openNewBrowserWindow();
+  let loadedPromise = BrowserTestUtils.browserLoaded(
+    newWin.gBrowser.selectedBrowser
+  );
+  BrowserTestUtils.loadURIString(
+    newWin.gBrowser.selectedBrowser,
+    "about:mozilla"
+  );
+  await loadedPromise;
+  await BrowserTestUtils.closeWindow(newWin);
+
+  await openHistoryPanel();
+
+  await BrowserTestUtils.waitForCondition(
+    () => !recentlyClosedWindows.getAttribute("disabled"),
+    "Waiting for button to be enabled"
+  );
+  Assert.ok(
+    !recentlyClosedTabs.getAttribute("disabled"),
+    "Recently closed tabs is available"
+  );
+  Assert.ok(
+    !recentlyClosedWindows.getAttribute("disabled"),
+    "Recently closed windows is available"
+  );
+
+  await hideHistoryPanel();
+});
+
+add_task(async function testRecentlyClosedTabsDisabledPersists() {
+  info("Check history recently closed tabs/windows section");
+
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  // We need to make sure the history is cleared before starting the test
+  await Sanitizer.sanitize(["history"]);
+
+  await openHistoryPanel();
+
+  let recentlyClosedTabs = document.getElementById("appMenuRecentlyClosedTabs");
+  Assert.ok(
+    recentlyClosedTabs.getAttribute("disabled"),
+    "Recently closed tabs button disabled"
+  );
+
+  await hideHistoryPanel();
+
+  let newWin = await BrowserTestUtils.openNewBrowserWindow();
+
+  await openHistoryPanel(newWin.document);
+  recentlyClosedTabs = newWin.document.getElementById(
+    "appMenuRecentlyClosedTabs"
+  );
+  Assert.ok(
+    recentlyClosedTabs.getAttribute("disabled"),
+    "Recently closed tabs is disabled"
+  );
+
+  // We close the window without hiding the panel first, which used to interfere
+  // with populating the view subsequently.
+  await BrowserTestUtils.closeWindow(newWin);
+
+  newWin = await BrowserTestUtils.openNewBrowserWindow();
+  await openHistoryPanel(newWin.document);
+  recentlyClosedTabs = newWin.document.getElementById(
+    "appMenuRecentlyClosedTabs"
+  );
+  Assert.ok(
+    recentlyClosedTabs.getAttribute("disabled"),
+    "Recently closed tabs is disabled"
+  );
+  await hideHistoryPanel(newWin.document);
+  await BrowserTestUtils.closeWindow(newWin);
+});
+
+add_task(async function testRecentlyClosedWindows() {
+  // We need to make sure the history is cleared before starting the test
+  await Sanitizer.sanitize(["history"]);
+
+  // Open and close a new window.
+  let newWin = await BrowserTestUtils.openNewBrowserWindow();
+  let loadedPromise = BrowserTestUtils.browserLoaded(
+    newWin.gBrowser.selectedBrowser
+  );
+  BrowserTestUtils.loadURIString(
+    newWin.gBrowser.selectedBrowser,
+    "https://example.com"
+  );
+  await loadedPromise;
+  await BrowserTestUtils.closeWindow(newWin);
+
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+  await openHistoryPanel();
+
+  // Open the "Recently closed windows" panel.
+  document.getElementById("appMenuRecentlyClosedWindows").click();
+
+  let winPanel = document.getElementById(
+    "appMenu-library-recentlyClosedWindows"
+  );
+  await BrowserTestUtils.waitForEvent(winPanel, "ViewShown");
+  ok(true, "Opened 'Recently closed windows' panel");
+
+  // Click the first toolbar button in the panel.
+  let panelBody = winPanel.querySelector(".panel-subview-body");
+  let toolbarButton = panelBody.querySelector("toolbarbutton");
+  let newWindowPromise = BrowserTestUtils.waitForNewWindow();
+  EventUtils.sendMouseEvent({ type: "click" }, toolbarButton, window);
+
+  newWin = await newWindowPromise;
+  is(
+    newWin.gBrowser.currentURI.spec,
+    "https://example.com/",
+    "Opened correct URL"
+  );
+  is(gBrowser.tabs.length, 1, "Did not open new tabs");
+
+  await BrowserTestUtils.closeWindow(newWin);
+});
diff --git a/browser/components/customizableui/test/browser_history_recently_closed_middleclick.js b/browser/components/customizableui/test/browser_history_recently_closed_middleclick.js
new file mode 100644
index 0000000000..309554ce23
--- /dev/null
+++ b/browser/components/customizableui/test/browser_history_recently_closed_middleclick.js
@@ -0,0 +1,106 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+// Verifies that middle-clicking "Recently Closed Tabs" in both history
+// menus works as expected.
+
+const URLS = [
+  "http://example.com/",
+  "http://example.org/",
+  "http://example.net/",
+];
+
+async function setupTest() {
+  // Navigate the initial tab to ensure that it won't be reused for the tab
+  // that will be reopened.
+  let loadPromise = BrowserTestUtils.browserLoaded(gBrowser.selectedBrowser);
+  BrowserTestUtils.loadURIString(
+    gBrowser.selectedBrowser,
+    "https://example.com"
+  );
+  await loadPromise;
+
+  // Populate the recently closed tabs list.
+  for (let url of URLS) {
+    await BrowserTestUtils.openNewForegroundTab(gBrowser, url);
+  }
+  for (let i = 0; i < URLS.length; i++) {
+    gBrowser.removeTab(gBrowser.selectedTab);
+  }
+
+  return gBrowser.tabs.length;
+}
+
+add_task(async function testMenubar() {
+  if (AppConstants.platform === "macosx") {
+    ok(true, "Can't open menu items on macOS");
+    return;
+  }
+
+  let nOpenTabs = await setupTest();
+
+  // Open the "History" menu.
+  let menu = document.getElementById("history-menu");
+  let popupPromise = BrowserTestUtils.waitForEvent(menu, "popupshown");
+  menu.open = true;
+  await popupPromise;
+  ok(true, "Opened 'History' menu");
+
+  // Open the "Recently Closed Tabs" submenu.
+  let undoMenu = document.getElementById("historyUndoMenu");
+  popupPromise = BrowserTestUtils.waitForEvent(undoMenu, "popupshown");
+  undoMenu.open = true;
+  let popupEvent = await popupPromise;
+  ok(true, "Opened 'Recently Closed Tabs' menu");
+
+  // And now middle-click the first item in that menu, and ensure that we're
+  // only opening a single new tab.
+  let menuitems = popupEvent.target.querySelectorAll("menuitem");
+  let newTabPromise = BrowserTestUtils.waitForNewTab(gBrowser, null, true);
+  popupEvent.target.activateItem(menuitems[0], { button: 1 });
+
+  let newTab = await newTabPromise;
+  is(newTab.linkedBrowser.currentURI.spec, URLS[0], "Opened correct URL");
+  is(gBrowser.tabs.length, nOpenTabs + 1, "Only opened 1 new tab");
+
+  gBrowser.removeTab(newTab);
+});
+
+add_task(async function testHistoryPanel() {
+  let nOpenTabs = await setupTest();
+
+  // Setup history panel.
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+  await openHistoryPanel();
+
+  // Open the "Recently closed tabs" panel.
+  let recentlyClosedTabs = document.getElementById("appMenuRecentlyClosedTabs");
+  recentlyClosedTabs.click();
+
+  let recentlyClosedTabsPanel = document.getElementById(
+    "appMenu-library-recentlyClosedTabs"
+  );
+  await BrowserTestUtils.waitForEvent(recentlyClosedTabsPanel, "ViewShown");
+  ok(true, "Opened 'Recently closed tabs' panel");
+
+  let panelBody = recentlyClosedTabsPanel.querySelector(".panel-subview-body");
+  let toolbarButtons = panelBody.querySelectorAll("toolbarbutton");
+
+  // Middle-click the first toolbar button in the panel.
+  let newTabPromise = BrowserTestUtils.waitForNewTab(gBrowser, null, true);
+  EventUtils.sendMouseEvent(
+    { type: "click", button: 1 },
+    toolbarButtons[0],
+    window
+  );
+
+  let newTab = await newTabPromise;
+  is(newTab.linkedBrowser.currentURI.spec, URLS[0], "Opened correct URL");
+  is(gBrowser.tabs.length, nOpenTabs + 1, "Only opened 1 new tab");
+
+  gBrowser.removeTab(newTab);
+});
diff --git a/browser/components/customizableui/test/browser_history_restore_session.js b/browser/components/customizableui/test/browser_history_restore_session.js
new file mode 100644
index 0000000000..a8b9529209
--- /dev/null
+++ b/browser/components/customizableui/test/browser_history_restore_session.js
@@ -0,0 +1,52 @@
+/* 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 testRestoreSession() {
+  info("Check history panel's restore previous session button");
+
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  CustomizableUI.addWidgetToArea(
+    "history-panelmenu",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  registerCleanupFunction(() => CustomizableUI.reset());
+
+  // We need to make sure the history is cleared before starting the test
+  await Sanitizer.sanitize(["history"]);
+
+  await openHistoryPanel(win.document);
+
+  let restorePrevSessionBtn = win.document.getElementById(
+    "appMenu-restoreSession"
+  );
+
+  Assert.ok(
+    restorePrevSessionBtn.hidden,
+    "Restore previous session button is not visible"
+  );
+  await hideHistoryPanel(win.document);
+
+  BrowserTestUtils.addTab(win.gBrowser, "about:mozilla");
+  await BrowserTestUtils.closeWindow(win);
+
+  win = await BrowserTestUtils.openNewBrowserWindow();
+
+  let lastSession = ChromeUtils.importESModule(
+    "resource:///modules/sessionstore/SessionStore.sys.mjs"
+  )._LastSession;
+  lastSession.setState(true);
+
+  await openHistoryPanel(win.document);
+
+  restorePrevSessionBtn = win.document.getElementById("appMenu-restoreSession");
+  Assert.ok(
+    !restorePrevSessionBtn.hidden,
+    "Restore previous session button is visible"
+  );
+
+  await hideHistoryPanel(win.document);
+  await BrowserTestUtils.closeWindow(win);
+});
diff --git a/browser/components/customizableui/test/browser_insert_before_moved_node.js b/browser/components/customizableui/test/browser_insert_before_moved_node.js
new file mode 100644
index 0000000000..611f4e3ce0
--- /dev/null
+++ b/browser/components/customizableui/test/browser_insert_before_moved_node.js
@@ -0,0 +1,51 @@
+"use strict";
+
+/**
+ * Check inserting before a node that has moved from the toolbar into a
+ * non-customizable bit of the browser works.
+ */
+add_task(async function () {
+  for (let toolbar of ["nav-bar", "TabsToolbar"]) {
+    CustomizableUI.createWidget({
+      id: "real-button",
+      label: "test real button",
+    });
+    CustomizableUI.addWidgetToArea("real-button", toolbar);
+    CustomizableUI.addWidgetToArea("moved-button-not-here", toolbar);
+    let placements = CustomizableUI.getWidgetIdsInArea(toolbar);
+    Assert.deepEqual(
+      placements.slice(-2),
+      ["real-button", "moved-button-not-here"],
+      "Should have correct placements"
+    );
+    let otherButton = document.createXULElement("toolbarbutton");
+    otherButton.id = "moved-button-not-here";
+    if (toolbar == "nav-bar") {
+      gURLBar.textbox.parentNode.appendChild(otherButton);
+    } else {
+      gBrowser.tabContainer.appendChild(otherButton);
+    }
+    CustomizableUI.destroyWidget("real-button");
+    CustomizableUI.createWidget({
+      id: "real-button",
+      label: "test real button",
+    });
+
+    let button = document.getElementById("real-button");
+    ok(button, "Button should exist");
+    if (button) {
+      let expectedContainer = CustomizableUI.getCustomizationTarget(
+        document.getElementById(toolbar)
+      );
+      is(
+        button.parentNode,
+        expectedContainer,
+        "Button should be in the toolbar"
+      );
+    }
+
+    CustomizableUI.destroyWidget("real-button");
+    otherButton.remove();
+    CustomizableUI.reset();
+  }
+});
diff --git a/browser/components/customizableui/test/browser_menubar_visibility.js b/browser/components/customizableui/test/browser_menubar_visibility.js
new file mode 100644
index 0000000000..82f2959905
--- /dev/null
+++ b/browser/components/customizableui/test/browser_menubar_visibility.js
@@ -0,0 +1,66 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check that menubar visibility is propagated correctly to new windows.
+ */
+add_task(async function test_menubar_visbility() {
+  let menubar = document.getElementById("toolbar-menubar");
+  is(menubar.getAttribute("autohide"), "true", "Menubar should be autohiding");
+  registerCleanupFunction(() => {
+    Services.xulStore.removeValue(
+      AppConstants.BROWSER_CHROME_URL,
+      menubar.id,
+      "autohide"
+    );
+    menubar.setAttribute("autohide", "true");
+  });
+
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  EventUtils.synthesizeMouse(
+    document.getElementById("stop-reload-button"),
+    2,
+    2,
+    {
+      type: "contextmenu",
+      button: 2,
+    }
+  );
+  await shownPromise;
+  let attrChanged = BrowserTestUtils.waitForAttribute(
+    "autohide",
+    menubar,
+    "false"
+  );
+  EventUtils.synthesizeMouseAtCenter(
+    document.getElementById("toggle_toolbar-menubar"),
+    {}
+  );
+  await attrChanged;
+  contextMenu.hidePopup(); // to be safe.
+
+  is(
+    menubar.getAttribute("autohide"),
+    "false",
+    "Menubar should now be permanently visible."
+  );
+  let persistedValue = Services.xulStore.getValue(
+    AppConstants.BROWSER_CHROME_URL,
+    menubar.id,
+    "autohide"
+  );
+  is(persistedValue, "false", "New value should be persisted");
+
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+
+  is(
+    win.document.getElementById("toolbar-menubar").getAttribute("autohide"),
+    "false",
+    "Menubar should also be permanently visible in the new window."
+  );
+
+  await BrowserTestUtils.closeWindow(win);
+});
diff --git a/browser/components/customizableui/test/browser_newtab_button_customizemode.js b/browser/components/customizableui/test/browser_newtab_button_customizemode.js
new file mode 100644
index 0000000000..c30616f3a3
--- /dev/null
+++ b/browser/components/customizableui/test/browser_newtab_button_customizemode.js
@@ -0,0 +1,181 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Tests in this file check that user customizations to the tabstrip show
+ * the correct type of new tab button while the tabstrip isn't overflowing.
+ */
+
+const kGlobalNewTabButton = document.getElementById("new-tab-button");
+const kInnerNewTabButton = gBrowser.tabContainer.newTabButton;
+
+function assertNewTabButton(which) {
+  if (which == "global") {
+    isnot(
+      kGlobalNewTabButton.getBoundingClientRect().width,
+      0,
+      "main new tab button should be visible"
+    );
+    is(
+      kInnerNewTabButton.getBoundingClientRect().width,
+      0,
+      "inner new tab button should be hidden"
+    );
+  } else if (which == "inner") {
+    is(
+      kGlobalNewTabButton.getBoundingClientRect().width,
+      0,
+      "main new tab button should be hidden"
+    );
+    isnot(
+      kInnerNewTabButton.getBoundingClientRect().width,
+      0,
+      "inner new tab button should be visible"
+    );
+  } else {
+    ok(false, "Unexpected button: " + which);
+  }
+}
+
+/**
+ * Add and remove items *after* the new tab button in customize mode.
+ */
+add_task(async function addremove_after_newtab_customizemode() {
+  await startCustomizing();
+  await waitForElementShown(kGlobalNewTabButton);
+  simulateItemDrag(
+    document.getElementById("stop-reload-button"),
+    kGlobalNewTabButton,
+    "end"
+  );
+  ok(
+    gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should have the adjacent newtab attribute"
+  );
+  await endCustomizing();
+  assertNewTabButton("inner");
+
+  await startCustomizing();
+  let dropTarget = document.getElementById("forward-button");
+  await waitForElementShown(dropTarget);
+  simulateItemDrag(
+    document.getElementById("stop-reload-button"),
+    dropTarget,
+    "end"
+  );
+  ok(
+    gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should still have the adjacent newtab attribute"
+  );
+  await endCustomizing();
+  assertNewTabButton("inner");
+  ok(CustomizableUI.inDefaultState, "Should be in default state");
+});
+
+/**
+ * Add and remove items *before* the new tab button in customize mode.
+ */
+add_task(async function addremove_before_newtab_customizemode() {
+  await startCustomizing();
+  await waitForElementShown(kGlobalNewTabButton);
+  simulateItemDrag(
+    document.getElementById("stop-reload-button"),
+    kGlobalNewTabButton,
+    "start"
+  );
+  ok(
+    !gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should no longer have the adjacent newtab attribute"
+  );
+  await endCustomizing();
+  assertNewTabButton("global");
+  await startCustomizing();
+  let dropTarget = document.getElementById("forward-button");
+  await waitForElementShown(dropTarget);
+  simulateItemDrag(
+    document.getElementById("stop-reload-button"),
+    dropTarget,
+    "end"
+  );
+  ok(
+    gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should have the adjacent newtab attribute again"
+  );
+  await endCustomizing();
+  assertNewTabButton("inner");
+  ok(CustomizableUI.inDefaultState, "Should be in default state");
+});
+
+/**
+ * Add and remove items *after* the new tab button outside of customize mode.
+ */
+add_task(async function addremove_after_newtab_api() {
+  CustomizableUI.addWidgetToArea("stop-reload-button", "TabsToolbar");
+  ok(
+    gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should have the adjacent newtab attribute"
+  );
+  assertNewTabButton("inner");
+
+  CustomizableUI.reset();
+  ok(
+    gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should still have the adjacent newtab attribute"
+  );
+  assertNewTabButton("inner");
+  ok(CustomizableUI.inDefaultState, "Should be in default state");
+});
+
+/**
+ * Add and remove items *before* the new tab button outside of customize mode.
+ */
+add_task(async function addremove_before_newtab_api() {
+  let index =
+    CustomizableUI.getWidgetIdsInArea("TabsToolbar").indexOf("new-tab-button");
+  CustomizableUI.addWidgetToArea("stop-reload-button", "TabsToolbar", index);
+  ok(
+    !gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should no longer have the adjacent newtab attribute"
+  );
+  assertNewTabButton("global");
+
+  CustomizableUI.removeWidgetFromArea("stop-reload-button");
+  ok(
+    gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should have the adjacent newtab attribute again"
+  );
+  assertNewTabButton("inner");
+
+  CustomizableUI.reset();
+  ok(CustomizableUI.inDefaultState, "Should be in default state");
+});
+
+/**
+ * Reset to defaults in customize mode to see if that doesn't break things.
+ */
+add_task(async function reset_before_newtab_customizemode() {
+  await startCustomizing();
+  await waitForElementShown(kGlobalNewTabButton);
+  simulateItemDrag(
+    document.getElementById("stop-reload-button"),
+    kGlobalNewTabButton,
+    "start"
+  );
+  ok(
+    !gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should no longer have the adjacent newtab attribute"
+  );
+  await endCustomizing();
+  assertNewTabButton("global");
+  await startCustomizing();
+  await gCustomizeMode.reset();
+  ok(
+    gBrowser.tabContainer.hasAttribute("hasadjacentnewtabbutton"),
+    "tabs should have the adjacent newtab attribute again"
+  );
+  await endCustomizing();
+  assertNewTabButton("inner");
+  ok(CustomizableUI.inDefaultState, "Should be in default state");
+});
diff --git a/browser/components/customizableui/test/browser_open_from_popup.js b/browser/components/customizableui/test/browser_open_from_popup.js
new file mode 100644
index 0000000000..bf140fde79
--- /dev/null
+++ b/browser/components/customizableui/test/browser_open_from_popup.js
@@ -0,0 +1,24 @@
+"use strict";
+
+/**
+ * Check that opening customize mode in a popup opens it in the main window.
+ */
+add_task(async function open_customize_mode_from_popup() {
+  let promiseWindow = BrowserTestUtils.waitForNewWindow();
+  SpecialPowers.spawn(gBrowser.selectedBrowser, [], function () {
+    content.window.open("about:blank", "_blank", "height=300,toolbar=no");
+  });
+  let win = await promiseWindow;
+  let customizePromise = BrowserTestUtils.waitForEvent(
+    gNavToolbox,
+    "customizationready"
+  );
+  win.gCustomizeMode.enter();
+  await customizePromise;
+  ok(
+    document.documentElement.hasAttribute("customizing"),
+    "Should have opened customize mode in the parent window"
+  );
+  await endCustomizing();
+  await BrowserTestUtils.closeWindow(win);
+});
diff --git a/browser/components/customizableui/test/browser_open_in_lazy_tab.js b/browser/components/customizableui/test/browser_open_in_lazy_tab.js
new file mode 100644
index 0000000000..c18de67698
--- /dev/null
+++ b/browser/components/customizableui/test/browser_open_in_lazy_tab.js
@@ -0,0 +1,42 @@
+"use strict";
+
+/**
+ * Check that customize mode can be loaded in a lazy tab.
+ */
+add_task(async function open_customize_mode_in_lazy_tab() {
+  let tab = BrowserTestUtils.addTab(gBrowser, "about:blank", {
+    createLazyBrowser: true,
+  });
+  gCustomizeMode.setTab(tab);
+
+  is(tab.linkedPanel, "", "Tab should be lazy");
+
+  let title = gNavigatorBundle.getFormattedString("customizeMode.tabTitle", [
+    document.getElementById("bundle_brand").getString("brandShortName"),
+  ]);
+  is(tab.label, title, "Tab should have correct title");
+
+  let customizePromise = BrowserTestUtils.waitForEvent(
+    gNavToolbox,
+    "customizationready"
+  );
+  gCustomizeMode.enter();
+  await customizePromise;
+
+  is(
+    tab.getAttribute("customizemode"),
+    "true",
+    "Tab should be in customize mode"
+  );
+
+  let customizationContainer = document.getElementById(
+    "customization-container"
+  );
+  is(
+    customizationContainer.hidden,
+    false,
+    "Customization container should be visible"
+  );
+
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_overflow_use_subviews.js b/browser/components/customizableui/test/browser_overflow_use_subviews.js
new file mode 100644
index 0000000000..1e6e227364
--- /dev/null
+++ b/browser/components/customizableui/test/browser_overflow_use_subviews.js
@@ -0,0 +1,88 @@
+"use strict";
+
+const kOverflowPanel = document.getElementById("widget-overflow");
+
+var gOriginalWidth;
+async function stopOverflowing() {
+  kOverflowPanel.removeAttribute("animate");
+  window.resizeTo(gOriginalWidth, window.outerHeight);
+  await TestUtils.waitForCondition(
+    () => !document.getElementById("nav-bar").hasAttribute("overflowing")
+  );
+  CustomizableUI.reset();
+}
+
+registerCleanupFunction(stopOverflowing);
+
+/**
+ * This checks that subview-compatible items show up as subviews rather than
+ * re-anchored panels. If we ever remove the library widget, please
+ * replace this test with another subview - don't remove it.
+ */
+add_task(async function check_library_subview_in_overflow() {
+  kOverflowPanel.setAttribute("animate", "false");
+  gOriginalWidth = window.outerWidth;
+
+  CustomizableUI.addWidgetToArea("library-button", CustomizableUI.AREA_NAVBAR);
+
+  let navbar = document.getElementById(CustomizableUI.AREA_NAVBAR);
+  ok(
+    !navbar.hasAttribute("overflowing"),
+    "Should start with a non-overflowing toolbar."
+  );
+  window.resizeTo(kForceOverflowWidthPx, window.outerHeight);
+
+  await TestUtils.waitForCondition(() => navbar.hasAttribute("overflowing"));
+
+  let chevron = document.getElementById("nav-bar-overflow-button");
+  let shownPanelPromise = BrowserTestUtils.waitForEvent(
+    kOverflowPanel,
+    "ViewShown"
+  );
+  chevron.click();
+  await shownPanelPromise;
+
+  let button = document.getElementById("library-button");
+  button.click();
+
+  let libraryView = document.getElementById("appMenu-libraryView");
+  await BrowserTestUtils.waitForEvent(libraryView, "ViewShown");
+  let hasSubviews = !!kOverflowPanel.querySelector("panelmultiview");
+  let expectedPanel = hasSubviews
+    ? kOverflowPanel
+    : document.getElementById("customizationui-widget-panel");
+  is(libraryView.closest("panel"), expectedPanel, "Should be inside the panel");
+  expectedPanel.hidePopup();
+  await Promise.resolve(); // wait for popup to hide fully.
+  await stopOverflowing();
+});
+
+/**
+ * This checks that non-subview-compatible items still work correctly.
+ * Ideally we should make the downloads panel and bookmarks/library item
+ * proper subview items, then this test can go away, and potentially we can
+ * simplify some of the subview anchoring code.
+ */
+add_task(async function check_downloads_panel_in_overflow() {
+  let button = document.getElementById("downloads-button");
+  await gCustomizeMode.addToPanel(button);
+  await waitForOverflowButtonShown();
+
+  let chevron = document.getElementById("nav-bar-overflow-button");
+  let shownPanelPromise = promisePanelElementShown(window, kOverflowPanel);
+  chevron.click();
+  await shownPanelPromise;
+
+  button.click();
+  await TestUtils.waitForCondition(() => {
+    let panel = document.getElementById("downloadsPanel");
+    return panel && panel.state != "closed";
+  });
+  let downloadsPanel = document.getElementById("downloadsPanel");
+  isnot(
+    downloadsPanel.state,
+    "closed",
+    "Should be attempting to show the downloads panel."
+  );
+  downloadsPanel.hidePopup();
+});
diff --git a/browser/components/customizableui/test/browser_palette_labels.js b/browser/components/customizableui/test/browser_palette_labels.js
new file mode 100644
index 0000000000..42767a8ee2
--- /dev/null
+++ b/browser/components/customizableui/test/browser_palette_labels.js
@@ -0,0 +1,66 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check that all customizable buttons have labels and icons.
+ *
+ * This is primarily designed to ensure we don't end up with items without
+ * labels in customize mode. In the past, this has happened due to race
+ * conditions, where labels would be correct if and only if the item had
+ * already been moved into a toolbar or panel in the main UI before
+ * (forcing it to be constructed and any fluent identifiers to be localized
+ * and applied).
+ * We use a new window to ensure that earlier tests using some of the widgets
+ * in the palette do not influence our checks to see that such items get
+ * labels, "even" if the first time they're rendered is in customize mode's
+ * palette.
+ */
+add_task(async function test_all_buttons_have_labels() {
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  registerCleanupFunction(async () => {
+    await endCustomizing(win);
+    return BrowserTestUtils.closeWindow(win);
+  });
+  await startCustomizing(win);
+  let { palette } = win.gNavToolbox;
+  // Wait for things to paint.
+  await TestUtils.waitForCondition(() => {
+    return !!Array.from(palette.querySelectorAll(".toolbarbutton-icon")).filter(
+      n => {
+        let rect = n.getBoundingClientRect();
+        return rect.height > 0 && rect.width > 0;
+      }
+    ).length;
+  }, "Must start rendering icons.");
+
+  for (let wrapper of palette.children) {
+    if (wrapper.hasAttribute("title")) {
+      ok(true, wrapper.firstElementChild.id + " has a label.");
+    } else {
+      info(
+        `${wrapper.firstElementChild.id} doesn't seem to have a label, waiting.`
+      );
+      await BrowserTestUtils.waitForAttribute("title", wrapper);
+      ok(
+        wrapper.hasAttribute("title"),
+        wrapper.firstElementChild.id + " has a label."
+      );
+    }
+    let icons = Array.from(wrapper.querySelectorAll(".toolbarbutton-icon"));
+    // If there are icons, at least one must be visible
+    // (not everything necessarily has one, e.g. the search bar has no icon)
+    if (icons.length) {
+      let visibleIcons = icons.filter(n => {
+        let rect = n.getBoundingClientRect();
+        return rect.height > 0 && rect.width > 0;
+      });
+      Assert.greater(
+        visibleIcons.length,
+        0,
+        `${wrapper.firstElementChild.id} should have at least one visible icon.`
+      );
+    }
+  }
+});
diff --git a/browser/components/customizableui/test/browser_panelUINotifications.js b/browser/components/customizableui/test/browser_panelUINotifications.js
new file mode 100644
index 0000000000..818fcbad39
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panelUINotifications.js
@@ -0,0 +1,597 @@
+"use strict";
+
+const { AppMenuNotifications } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppMenuNotifications.sys.mjs"
+);
+
+/**
+ * Tests that when we click on the main call-to-action of the doorhanger, the provided
+ * action is called, and the doorhanger removed.
+ */
+add_task(async function testMainActionCalled() {
+  let options = {
+    gBrowser: window.gBrowser,
+    url: "about:blank",
+  };
+
+  await BrowserTestUtils.withNewTab(options, function (browser) {
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+    let mainActionCalled = false;
+    let mainAction = {
+      callback: () => {
+        mainActionCalled = true;
+      },
+    };
+    AppMenuNotifications.showNotification("update-manual", mainAction);
+
+    isnot(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is showing."
+    );
+    let notifications = [...PanelUI.notificationPanel.children].filter(
+      n => !n.hidden
+    );
+    is(
+      notifications.length,
+      1,
+      "PanelUI doorhanger is only displaying one notification."
+    );
+    let doorhanger = notifications[0];
+    is(
+      doorhanger.id,
+      "appMenu-update-manual-notification",
+      "PanelUI is displaying the update-manual notification."
+    );
+
+    let button = doorhanger.button;
+    button.click();
+
+    ok(mainActionCalled, "Main action callback was called");
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+    is(
+      PanelUI.menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status"
+    );
+  });
+});
+
+/**
+ * This tests that when we click the secondary action for a notification,
+ * it will display the badge for that notification on the PanelUI menu button.
+ * Once we click on this button, we should see an item in the menu which will
+ * call our main action.
+ */
+add_task(async function testSecondaryActionWorkflow() {
+  let options = {
+    gBrowser: window.gBrowser,
+    url: "about:blank",
+  };
+
+  await BrowserTestUtils.withNewTab(options, async function (browser) {
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+
+    let mainActionCalled = false;
+    let mainAction = {
+      callback: () => {
+        mainActionCalled = true;
+      },
+    };
+    AppMenuNotifications.showNotification("update-manual", mainAction);
+
+    isnot(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is showing."
+    );
+    let notifications = [...PanelUI.notificationPanel.children].filter(
+      n => !n.hidden
+    );
+    is(
+      notifications.length,
+      1,
+      "PanelUI doorhanger is only displaying one notification."
+    );
+    let doorhanger = notifications[0];
+    is(
+      doorhanger.id,
+      "appMenu-update-manual-notification",
+      "PanelUI is displaying the update-manual notification."
+    );
+
+    let secondaryActionButton = doorhanger.secondaryButton;
+    secondaryActionButton.click();
+
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-manual",
+      "Badge is displaying on PanelUI button."
+    );
+
+    await gCUITestUtils.openMainMenu();
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-manual",
+      "Badge is displaying on PanelUI button."
+    );
+    let menuItem = PanelUI.mainView.querySelector(".panel-banner-item");
+    is(
+      menuItem.getAttribute("data-l10n-id"),
+      "appmenuitem-banner-update-manual",
+      "Showing correct label"
+    );
+    is(menuItem.hidden, false, "update-manual menu item is showing.");
+
+    await gCUITestUtils.hideMainMenu();
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-manual",
+      "Badge is shown on PanelUI button."
+    );
+
+    await gCUITestUtils.openMainMenu();
+    menuItem.click();
+    ok(mainActionCalled, "Main action callback was called");
+
+    AppMenuNotifications.removeNotification(/.*/);
+  });
+});
+
+/**
+ * This tests that the PanelUI update downloading badge and banner
+ * notification are correctly displayed and that clicking the banner
+ * item calls the main action.
+ */
+add_task(async function testDownloadingBadge() {
+  let options = {
+    gBrowser: window.gBrowser,
+    url: "about:blank",
+  };
+
+  await BrowserTestUtils.withNewTab(options, async function (browser) {
+    let mainActionCalled = false;
+    let mainAction = {
+      callback: () => {
+        mainActionCalled = true;
+      },
+    };
+    // The downloading notification is always displayed in a dismissed state.
+    AppMenuNotifications.showNotification(
+      "update-downloading",
+      mainAction,
+      undefined,
+      { dismissed: true }
+    );
+    is(PanelUI.notificationPanel.state, "closed", "doorhanger is closed.");
+
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-downloading",
+      "Downloading badge is displaying on PanelUI button."
+    );
+
+    await gCUITestUtils.openMainMenu();
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-downloading",
+      "Downloading badge is displaying on PanelUI button."
+    );
+    let menuItem = PanelUI.mainView.querySelector(".panel-banner-item");
+    is(
+      menuItem.getAttribute("data-l10n-id"),
+      "appmenuitem-banner-update-downloading",
+      "Showing correct label (downloading)"
+    );
+    is(menuItem.hidden, false, "update-downloading menu item is showing.");
+
+    await gCUITestUtils.hideMainMenu();
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-downloading",
+      "Downloading badge is shown on PanelUI button."
+    );
+
+    await gCUITestUtils.openMainMenu();
+    menuItem.click();
+    ok(mainActionCalled, "Main action callback was called");
+
+    AppMenuNotifications.removeNotification(/.*/);
+  });
+});
+
+/**
+ * We want to ensure a few things with this:
+ * - Adding a doorhanger will make a badge disappear
+ * - once the notification for the doorhanger is resolved (removed, not just dismissed),
+ *   then we display any other badges that are remaining.
+ */
+add_task(async function testInteractionWithBadges() {
+  await BrowserTestUtils.withNewTab("about:blank", async function (browser) {
+    // Remove the fxa toolbar button from the navbar to ensure the notification
+    // is displayed on the app menu button.
+    let { CustomizableUI } = ChromeUtils.importESModule(
+      "resource:///modules/CustomizableUI.sys.mjs"
+    );
+    CustomizableUI.removeWidgetFromArea("fxa-toolbar-menu-button");
+
+    AppMenuNotifications.showBadgeOnlyNotification("fxa-needs-authentication");
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "fxa-needs-authentication",
+      "Fxa badge is shown on PanelUI button."
+    );
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+
+    let mainActionCalled = false;
+    let mainAction = {
+      callback: () => {
+        mainActionCalled = true;
+      },
+    };
+    AppMenuNotifications.showNotification("update-manual", mainAction);
+
+    isnot(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "fxa-needs-authentication",
+      "Fxa badge is hidden on PanelUI button."
+    );
+    isnot(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is showing."
+    );
+    let notifications = [...PanelUI.notificationPanel.children].filter(
+      n => !n.hidden
+    );
+    is(
+      notifications.length,
+      1,
+      "PanelUI doorhanger is only displaying one notification."
+    );
+    let doorhanger = notifications[0];
+    is(
+      doorhanger.id,
+      "appMenu-update-manual-notification",
+      "PanelUI is displaying the update-manual notification."
+    );
+
+    let secondaryActionButton = doorhanger.secondaryButton;
+    secondaryActionButton.click();
+
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-manual",
+      "Badge is displaying on PanelUI button."
+    );
+
+    await gCUITestUtils.openMainMenu();
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-manual",
+      "Badge is displaying on PanelUI button."
+    );
+    let menuItem = PanelUI.mainView.querySelector(".panel-banner-item");
+    is(
+      menuItem.getAttribute("data-l10n-id"),
+      "appmenuitem-banner-update-manual",
+      "Showing correct label"
+    );
+    is(menuItem.hidden, false, "update-manual menu item is showing.");
+
+    menuItem.click();
+    ok(mainActionCalled, "Main action callback was called");
+
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "fxa-needs-authentication",
+      "Fxa badge is shown on PanelUI button."
+    );
+    AppMenuNotifications.removeNotification(/.*/);
+    is(
+      PanelUI.menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status"
+    );
+  });
+});
+
+/**
+ * This tests that adding a badge will not dismiss any existing doorhangers.
+ */
+add_task(async function testAddingBadgeWhileDoorhangerIsShowing() {
+  await BrowserTestUtils.withNewTab("about:blank", function (browser) {
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+    let mainActionCalled = false;
+    let mainAction = {
+      callback: () => {
+        mainActionCalled = true;
+      },
+    };
+    AppMenuNotifications.showNotification("update-manual", mainAction);
+    AppMenuNotifications.showBadgeOnlyNotification("fxa-needs-authentication");
+
+    isnot(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "fxa-needs-authentication",
+      "Fxa badge is hidden on PanelUI button."
+    );
+    isnot(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is showing."
+    );
+    let notifications = [...PanelUI.notificationPanel.children].filter(
+      n => !n.hidden
+    );
+    is(
+      notifications.length,
+      1,
+      "PanelUI doorhanger is only displaying one notification."
+    );
+    let doorhanger = notifications[0];
+    is(
+      doorhanger.id,
+      "appMenu-update-manual-notification",
+      "PanelUI is displaying the update-manual notification."
+    );
+
+    let mainActionButton = doorhanger.button;
+    mainActionButton.click();
+
+    ok(mainActionCalled, "Main action callback was called");
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "fxa-needs-authentication",
+      "Fxa badge is shown on PanelUI button."
+    );
+    AppMenuNotifications.removeNotification(/.*/);
+    is(
+      PanelUI.menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status"
+    );
+  });
+});
+
+/**
+ * Tests that badges operate like a stack.
+ */
+add_task(async function testMultipleBadges() {
+  await BrowserTestUtils.withNewTab("about:blank", async function (browser) {
+    let doc = browser.ownerDocument;
+    let menuButton = doc.getElementById("PanelUI-menu-button");
+
+    is(
+      menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status"
+    );
+    is(
+      menuButton.hasAttribute("badge"),
+      false,
+      "Should not have the badge attribute set"
+    );
+
+    AppMenuNotifications.showBadgeOnlyNotification("fxa-needs-authentication");
+    is(
+      menuButton.getAttribute("badge-status"),
+      "fxa-needs-authentication",
+      "Should have fxa-needs-authentication badge status"
+    );
+
+    AppMenuNotifications.showBadgeOnlyNotification("update-succeeded");
+    is(
+      menuButton.getAttribute("badge-status"),
+      "update-succeeded",
+      "Should have update-succeeded badge status (update > fxa)"
+    );
+
+    AppMenuNotifications.showBadgeOnlyNotification("update-failed");
+    is(
+      menuButton.getAttribute("badge-status"),
+      "update-failed",
+      "Should have update-failed badge status"
+    );
+
+    AppMenuNotifications.removeNotification(/^update-/);
+    is(
+      menuButton.getAttribute("badge-status"),
+      "fxa-needs-authentication",
+      "Should have fxa-needs-authentication badge status"
+    );
+
+    AppMenuNotifications.removeNotification(/^fxa-/);
+    is(
+      menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status"
+    );
+
+    await gCUITestUtils.openMainMenu();
+    is(
+      menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status (Hamburger menu opened)"
+    );
+    await gCUITestUtils.hideMainMenu();
+
+    AppMenuNotifications.showBadgeOnlyNotification("fxa-needs-authentication");
+    AppMenuNotifications.showBadgeOnlyNotification("update-succeeded");
+    AppMenuNotifications.removeNotification(/.*/);
+    is(
+      menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status"
+    );
+  });
+});
+
+/**
+ * Tests that non-badges also operate like a stack.
+ */
+add_task(async function testMultipleNonBadges() {
+  await BrowserTestUtils.withNewTab("about:blank", async function (browser) {
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+
+    let updateManualAction = {
+      called: false,
+      callback: () => {
+        updateManualAction.called = true;
+      },
+    };
+    let updateRestartAction = {
+      called: false,
+      callback: () => {
+        updateRestartAction.called = true;
+      },
+    };
+
+    AppMenuNotifications.showNotification("update-manual", updateManualAction);
+
+    let notifications;
+    let doorhanger;
+
+    isnot(PanelUI.notificationPanel.state, "closed", "Doorhanger is showing.");
+    notifications = [...PanelUI.notificationPanel.children].filter(
+      n => !n.hidden
+    );
+    is(
+      notifications.length,
+      1,
+      "PanelUI doorhanger is only displaying one notification."
+    );
+    doorhanger = notifications[0];
+    is(
+      doorhanger.id,
+      "appMenu-update-manual-notification",
+      "PanelUI is displaying the update-manual notification."
+    );
+
+    AppMenuNotifications.showNotification(
+      "update-restart",
+      updateRestartAction
+    );
+
+    isnot(PanelUI.notificationPanel.state, "closed", "Doorhanger is showing.");
+    notifications = [...PanelUI.notificationPanel.children].filter(
+      n => !n.hidden
+    );
+    is(
+      notifications.length,
+      1,
+      "PanelUI doorhanger is only displaying one notification."
+    );
+    doorhanger = notifications[0];
+    is(
+      doorhanger.id,
+      "appMenu-update-restart-notification",
+      "PanelUI is displaying the update-restart notification."
+    );
+
+    let secondaryActionButton = doorhanger.secondaryButton;
+    secondaryActionButton.click();
+
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-restart",
+      "update-restart badge is displaying on PanelUI button."
+    );
+
+    await gCUITestUtils.openMainMenu();
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-restart",
+      "update-restart badge is displaying on PanelUI button."
+    );
+    let menuItem = PanelUI.mainView.querySelector(".panel-banner-item");
+    is(
+      menuItem.getAttribute("data-l10n-id"),
+      "appmenuitem-banner-update-restart",
+      "Showing correct label"
+    );
+    is(menuItem.hidden, false, "update-restart menu item is showing.");
+
+    menuItem.click();
+    ok(
+      updateRestartAction.called,
+      "update-restart main action callback was called"
+    );
+
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-manual",
+      "update-manual badge is displaying on PanelUI button."
+    );
+
+    await gCUITestUtils.openMainMenu();
+    is(
+      PanelUI.menuButton.getAttribute("badge-status"),
+      "update-manual",
+      "update-manual badge is displaying on PanelUI button."
+    );
+    is(
+      menuItem.getAttribute("data-l10n-id"),
+      "appmenuitem-banner-update-manual",
+      "Showing correct label"
+    );
+    is(menuItem.hidden, false, "update-manual menu item is showing.");
+
+    menuItem.click();
+    ok(
+      updateManualAction.called,
+      "update-manual main action callback was called"
+    );
+  });
+});
diff --git a/browser/components/customizableui/test/browser_panelUINotifications_bannerVisibility.js b/browser/components/customizableui/test/browser_panelUINotifications_bannerVisibility.js
new file mode 100644
index 0000000000..ff4c10ee11
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panelUINotifications_bannerVisibility.js
@@ -0,0 +1,139 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { AppMenuNotifications } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppMenuNotifications.sys.mjs"
+);
+
+/**
+ * The update banner should become visible when the badge-only notification is
+ * shown before opening the menu.
+ */
+add_task(async function testBannerVisibilityBeforeOpen() {
+  let newWin = await BrowserTestUtils.openNewBrowserWindow();
+
+  AppMenuNotifications.showBadgeOnlyNotification("update-restart");
+
+  let menuButton = newWin.document.getElementById("PanelUI-menu-button");
+  let shown = BrowserTestUtils.waitForEvent(
+    newWin.PanelUI.mainView,
+    "ViewShown"
+  );
+  menuButton.click();
+  await shown;
+
+  let banner = newWin.document.getElementById("appMenu-proton-update-banner");
+
+  let labelPromise = BrowserTestUtils.waitForMutationCondition(
+    banner,
+    { attributes: true, attributeFilter: ["label"] },
+    () => banner.hasAttribute("label")
+  );
+
+  ok(!banner.hidden, "Update banner should be shown");
+
+  await labelPromise;
+
+  ok(banner.getAttribute("label") != "", "Update banner should contain text");
+
+  AppMenuNotifications.removeNotification(/.*/);
+
+  await BrowserTestUtils.closeWindow(newWin);
+});
+
+/**
+ * The update banner should become visible when the badge-only notification is
+ * shown during the menu is opened.
+ */
+add_task(async function testBannerVisibilityDuringOpen() {
+  let newWin = await BrowserTestUtils.openNewBrowserWindow();
+
+  let menuButton = newWin.document.getElementById("PanelUI-menu-button");
+  let shown = BrowserTestUtils.waitForEvent(
+    newWin.PanelUI.mainView,
+    "ViewShown"
+  );
+  menuButton.click();
+  await shown;
+
+  let banner = newWin.document.getElementById("appMenu-proton-update-banner");
+  ok(
+    !banner.hasAttribute("label"),
+    "Update banner shouldn't contain text before notification"
+  );
+
+  let labelPromise = BrowserTestUtils.waitForMutationCondition(
+    banner,
+    { attributes: true, attributeFilter: ["label"] },
+    () => banner.hasAttribute("label")
+  );
+
+  AppMenuNotifications.showNotification("update-restart");
+
+  ok(!banner.hidden, "Update banner should be shown");
+
+  await labelPromise;
+
+  ok(banner.getAttribute("label") != "", "Update banner should contain text");
+
+  AppMenuNotifications.removeNotification(/.*/);
+
+  await BrowserTestUtils.closeWindow(newWin);
+});
+
+/**
+ * The update banner should become visible when the badge-only notification is
+ * shown after opening/closing the menu, so that the DOM tree is there but
+ * the menu is closed.
+ */
+add_task(async function testBannerVisibilityAfterClose() {
+  let newWin = await BrowserTestUtils.openNewBrowserWindow();
+
+  let menuButton = newWin.document.getElementById("PanelUI-menu-button");
+  let shown = BrowserTestUtils.waitForEvent(
+    newWin.PanelUI.mainView,
+    "ViewShown"
+  );
+  menuButton.click();
+  await shown;
+
+  ok(newWin.PanelUI.mainView.hasAttribute("visible"));
+
+  let banner = newWin.document.getElementById("appMenu-proton-update-banner");
+
+  ok(banner.hidden, "Update banner should be hidden before notification");
+  ok(
+    !banner.hasAttribute("label"),
+    "Update banner shouldn't contain text before notification"
+  );
+
+  let labelPromise = BrowserTestUtils.waitForMutationCondition(
+    banner,
+    { attributes: true, attributeFilter: ["label"] },
+    () => banner.hasAttribute("label")
+  );
+
+  let hidden = BrowserTestUtils.waitForCondition(() => {
+    return !newWin.PanelUI.mainView.hasAttribute("visible");
+  });
+  menuButton.click();
+  await hidden;
+
+  AppMenuNotifications.showBadgeOnlyNotification("update-restart");
+
+  shown = BrowserTestUtils.waitForEvent(newWin.PanelUI.mainView, "ViewShown");
+  menuButton.click();
+  await shown;
+
+  ok(!banner.hidden, "Update banner should be shown");
+
+  await labelPromise;
+
+  ok(banner.getAttribute("label") != "", "Update banner should contain text");
+
+  AppMenuNotifications.removeNotification(/.*/);
+
+  await BrowserTestUtils.closeWindow(newWin);
+});
diff --git a/browser/components/customizableui/test/browser_panelUINotifications_fullscreen.js b/browser/components/customizableui/test/browser_panelUINotifications_fullscreen.js
new file mode 100644
index 0000000000..4b3340696b
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panelUINotifications_fullscreen.js
@@ -0,0 +1,92 @@
+"use strict";
+
+const { AppMenuNotifications } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppMenuNotifications.sys.mjs"
+);
+
+add_task(async function testFullscreen() {
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+  let mainActionCalled = false;
+  let mainAction = {
+    callback: () => {
+      mainActionCalled = true;
+    },
+  };
+  AppMenuNotifications.showNotification("update-manual", mainAction);
+
+  isnot(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is showing."
+  );
+  let notifications = [...PanelUI.notificationPanel.children].filter(
+    n => !n.hidden
+  );
+  is(
+    notifications.length,
+    1,
+    "PanelUI doorhanger is only displaying one notification."
+  );
+  let doorhanger = notifications[0];
+  is(
+    doorhanger.id,
+    "appMenu-update-manual-notification",
+    "PanelUI is displaying the update-manual notification."
+  );
+
+  let popuphiddenPromise = BrowserTestUtils.waitForEvent(
+    PanelUI.notificationPanel,
+    "popuphidden"
+  );
+  document.documentElement.focus();
+  EventUtils.synthesizeKey("KEY_F11");
+  await popuphiddenPromise;
+  await new Promise(executeSoon);
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+
+  FullScreen.showNavToolbox();
+  is(
+    PanelUI.menuButton.getAttribute("badge-status"),
+    "update-manual",
+    "Badge is displaying on PanelUI button."
+  );
+
+  let popupshownPromise = BrowserTestUtils.waitForEvent(
+    PanelUI.notificationPanel,
+    "popupshown"
+  );
+  EventUtils.synthesizeKey("KEY_F11");
+  await popupshownPromise;
+  await new Promise(executeSoon);
+  isnot(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is showing."
+  );
+  isnot(
+    PanelUI.menuButton.getAttribute("badge-status"),
+    "update-manual",
+    "Badge is not displaying on PanelUI button."
+  );
+
+  doorhanger.button.click();
+  ok(mainActionCalled, "Main action callback was called");
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+  is(
+    PanelUI.menuButton.hasAttribute("badge-status"),
+    false,
+    "Should not have a badge status"
+  );
+});
diff --git a/browser/components/customizableui/test/browser_panelUINotifications_fullscreen_noAutoHideToolbar.js b/browser/components/customizableui/test/browser_panelUINotifications_fullscreen_noAutoHideToolbar.js
new file mode 100644
index 0000000000..853c39e89f
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panelUINotifications_fullscreen_noAutoHideToolbar.js
@@ -0,0 +1,145 @@
+"use strict";
+
+// This test tends to trigger a race in the fullscreen time telemetry,
+// where the fullscreen enter and fullscreen exit events (which use the
+// same histogram ID) overlap. That causes TelemetryStopwatch to log an
+// error.
+SimpleTest.ignoreAllUncaughtExceptions(true);
+
+const { AppMenuNotifications } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppMenuNotifications.sys.mjs"
+);
+
+function waitForDocshellActivated() {
+  return SpecialPowers.spawn(gBrowser.selectedBrowser, [], async function () {
+    // Setting docshell activated/deactivated will trigger visibility state
+    // changes to relevant state ("visible" or "hidden"). AFAIK, there is no
+    // such event notifying docshell is being activated, so I use
+    // "visibilitychange" event rather than polling the isActive flag.
+    await ContentTaskUtils.waitForEvent(
+      content.document,
+      "visibilitychange",
+      true /* capture */,
+      aEvent => {
+        return content.browsingContext.isActive;
+      }
+    );
+  });
+}
+
+function waitForFullscreen() {
+  return Promise.all([
+    BrowserTestUtils.waitForEvent(window, "fullscreen"),
+    // In the platforms that support reporting occlusion state (e.g. Mac),
+    // enter/exit fullscreen mode will trigger docshell being set to non-activate
+    // and then set to activate back again. For those platforms, we should wait
+    // until the docshell has been activated again before starting next test,
+    // otherwise, the fullscreen request might be denied.
+    Services.appinfo.OS === "Darwin"
+      ? waitForDocshellActivated()
+      : Promise.resolve(),
+  ]);
+}
+
+add_task(async function testFullscreen() {
+  if (Services.appinfo.OS !== "Darwin") {
+    await SpecialPowers.pushPrefEnv({
+      set: [["browser.fullscreen.autohide", false]],
+    });
+  }
+
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+  let mainActionCalled = false;
+  let mainAction = {
+    callback: () => {
+      mainActionCalled = true;
+    },
+  };
+  AppMenuNotifications.showNotification("update-manual", mainAction);
+  await BrowserTestUtils.waitForEvent(PanelUI.notificationPanel, "popupshown");
+
+  isnot(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is showing."
+  );
+  let notifications = [...PanelUI.notificationPanel.children].filter(
+    n => !n.hidden
+  );
+  is(
+    notifications.length,
+    1,
+    "PanelUI doorhanger is only displaying one notification."
+  );
+  let doorhanger = notifications[0];
+  is(
+    doorhanger.id,
+    "appMenu-update-manual-notification",
+    "PanelUI is displaying the update-manual notification."
+  );
+
+  let fullscreenPromise = waitForFullscreen();
+  EventUtils.synthesizeKey("KEY_F11");
+  await fullscreenPromise;
+  isnot(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is still showing after entering fullscreen."
+  );
+
+  let popuphiddenPromise = BrowserTestUtils.waitForEvent(
+    PanelUI.notificationPanel,
+    "popuphidden"
+  );
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async () => {
+    content.document.documentElement.requestFullscreen();
+  });
+  await popuphiddenPromise;
+  await new Promise(executeSoon);
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is hidden after entering DOM fullscreen."
+  );
+
+  let popupshownPromise = BrowserTestUtils.waitForEvent(
+    PanelUI.notificationPanel,
+    "popupshown"
+  );
+  await SpecialPowers.spawn(gBrowser.selectedBrowser, [], async () => {
+    content.document.exitFullscreen();
+  });
+  await popupshownPromise;
+  await new Promise(executeSoon);
+  isnot(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is shown after exiting DOM fullscreen."
+  );
+  isnot(
+    PanelUI.menuButton.getAttribute("badge-status"),
+    "update-manual",
+    "Badge is not displaying on PanelUI button."
+  );
+
+  doorhanger.button.click();
+  ok(mainActionCalled, "Main action callback was called");
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+  is(
+    PanelUI.menuButton.hasAttribute("badge-status"),
+    false,
+    "Should not have a badge status"
+  );
+
+  fullscreenPromise = BrowserTestUtils.waitForEvent(window, "fullscreen");
+  EventUtils.synthesizeKey("KEY_F11");
+  await fullscreenPromise;
+});
diff --git a/browser/components/customizableui/test/browser_panelUINotifications_modals.js b/browser/components/customizableui/test/browser_panelUINotifications_modals.js
new file mode 100644
index 0000000000..87be14fcee
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panelUINotifications_modals.js
@@ -0,0 +1,87 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+const { AppMenuNotifications } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppMenuNotifications.sys.mjs"
+);
+
+add_task(async function testModals() {
+  await SpecialPowers.pushPrefEnv({
+    set: [["prompts.windowPromptSubDialog", true]],
+  });
+
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+  let mainActionCalled = false;
+  let mainAction = {
+    callback: () => {
+      mainActionCalled = true;
+    },
+  };
+  AppMenuNotifications.showNotification("update-manual", mainAction);
+
+  isnot(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is showing."
+  );
+  let notifications = [...PanelUI.notificationPanel.children].filter(
+    n => !n.hidden
+  );
+  is(
+    notifications.length,
+    1,
+    "PanelUI doorhanger is only displaying one notification."
+  );
+  let doorhanger = notifications[0];
+  is(
+    doorhanger.id,
+    "appMenu-update-manual-notification",
+    "PanelUI is displaying the update-manual notification."
+  );
+
+  let popuphiddenPromise = BrowserTestUtils.waitForEvent(
+    PanelUI.notificationPanel,
+    "popuphidden"
+  );
+
+  let dialogPromise = BrowserTestUtils.promiseAlertDialogOpen("accept");
+  Services.prompt.asyncAlert(
+    window.browsingContext,
+    Services.prompt.MODAL_TYPE_INTERNAL_WINDOW,
+    "Test alert",
+    "Test alert description"
+  );
+  await popuphiddenPromise;
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+
+  let popupshownPromise = BrowserTestUtils.waitForEvent(
+    PanelUI.notificationPanel,
+    "popupshown"
+  );
+
+  await dialogPromise;
+  await popupshownPromise;
+  isnot(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is showing."
+  );
+
+  doorhanger.button.click();
+  ok(mainActionCalled, "Main action callback was called");
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "update-manual doorhanger is closed."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_panelUINotifications_multiWindow.js b/browser/components/customizableui/test/browser_panelUINotifications_multiWindow.js
new file mode 100644
index 0000000000..29470a4cbb
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panelUINotifications_multiWindow.js
@@ -0,0 +1,206 @@
+"use strict";
+
+const { AppMenuNotifications } = ChromeUtils.importESModule(
+  "resource://gre/modules/AppMenuNotifications.sys.mjs"
+);
+
+/**
+ * Tests that when we try to show a notification in a background window, it
+ * does not display until the window comes back into the foreground. However,
+ * it should display a badge.
+ */
+add_task(async function testDoesNotShowDoorhangerForBackgroundWindow() {
+  let options = {
+    gBrowser: window.gBrowser,
+    url: "about:blank",
+  };
+
+  await BrowserTestUtils.withNewTab(options, async function (browser) {
+    let win = await BrowserTestUtils.openNewBrowserWindow();
+    await SimpleTest.promiseFocus(win);
+    let mainActionCalled = false;
+    let mainAction = {
+      callback: () => {
+        mainActionCalled = true;
+      },
+    };
+    AppMenuNotifications.showNotification("update-manual", mainAction);
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "The background window's doorhanger is closed."
+    );
+    is(
+      PanelUI.menuButton.hasAttribute("badge-status"),
+      true,
+      "The background window has a badge."
+    );
+
+    await BrowserTestUtils.closeWindow(win);
+    await SimpleTest.promiseFocus(window);
+    isnot(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is showing."
+    );
+    let notifications = [...PanelUI.notificationPanel.children].filter(
+      n => !n.hidden
+    );
+    is(
+      notifications.length,
+      1,
+      "PanelUI doorhanger is only displaying one notification."
+    );
+    let doorhanger = notifications[0];
+    is(
+      doorhanger.id,
+      "appMenu-update-manual-notification",
+      "PanelUI is displaying the update-manual notification."
+    );
+
+    let button = doorhanger.button;
+    button.click();
+
+    ok(mainActionCalled, "Main action callback was called");
+    is(
+      PanelUI.notificationPanel.state,
+      "closed",
+      "update-manual doorhanger is closed."
+    );
+    is(
+      PanelUI.menuButton.hasAttribute("badge-status"),
+      false,
+      "Should not have a badge status"
+    );
+  });
+});
+
+/**
+ * Tests that when we try to show a notification in a background window and in
+ * a foreground window, if the foreground window's main action is called, the
+ * background window's doorhanger will be removed.
+ */
+add_task(
+  async function testBackgroundWindowNotificationsAreRemovedByForeground() {
+    let options = {
+      gBrowser: window.gBrowser,
+      url: "about:blank",
+    };
+
+    await BrowserTestUtils.withNewTab(options, async function (browser) {
+      let win = await BrowserTestUtils.openNewBrowserWindow();
+      await SimpleTest.promiseFocus(win);
+      AppMenuNotifications.showNotification("update-manual", { callback() {} });
+      let notifications = [...win.PanelUI.notificationPanel.children].filter(
+        n => !n.hidden
+      );
+      is(
+        notifications.length,
+        1,
+        "PanelUI doorhanger is only displaying one notification."
+      );
+      let doorhanger = notifications[0];
+      doorhanger.button.click();
+
+      await BrowserTestUtils.closeWindow(win);
+      await SimpleTest.promiseFocus(window);
+
+      is(
+        PanelUI.notificationPanel.state,
+        "closed",
+        "update-manual doorhanger is closed."
+      );
+      is(
+        PanelUI.menuButton.hasAttribute("badge-status"),
+        false,
+        "Should not have a badge status"
+      );
+    });
+  }
+);
+
+/**
+ * Tests that when we try to show a notification in a background window and in
+ * a foreground window, if the foreground window's doorhanger is dismissed,
+ * the background window's doorhanger will also be dismissed once the window
+ * regains focus.
+ */
+add_task(
+  async function testBackgroundWindowNotificationsAreDismissedByForeground() {
+    let options = {
+      gBrowser: window.gBrowser,
+      url: "about:blank",
+    };
+
+    await BrowserTestUtils.withNewTab(options, async function (browser) {
+      let win = await BrowserTestUtils.openNewBrowserWindow();
+      await SimpleTest.promiseFocus(win);
+      AppMenuNotifications.showNotification("update-manual", { callback() {} });
+      let notifications = [...win.PanelUI.notificationPanel.children].filter(
+        n => !n.hidden
+      );
+      is(
+        notifications.length,
+        1,
+        "PanelUI doorhanger is only displaying one notification."
+      );
+      let doorhanger = notifications[0];
+      let button = doorhanger.secondaryButton;
+      button.click();
+
+      await BrowserTestUtils.closeWindow(win);
+      await SimpleTest.promiseFocus(window);
+
+      is(
+        PanelUI.notificationPanel.state,
+        "closed",
+        "The background window's doorhanger is closed."
+      );
+      is(
+        PanelUI.menuButton.hasAttribute("badge-status"),
+        true,
+        "The dismissed notification should still have a badge status"
+      );
+
+      AppMenuNotifications.removeNotification(/.*/);
+    });
+  }
+);
+
+/**
+ * Tests that when we open a new window while a notification is showing, the
+ * notification also shows on the new window.
+ */
+add_task(async function testOpenWindowAfterShowingNotification() {
+  AppMenuNotifications.showNotification("update-manual", { callback() {} });
+
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  await SimpleTest.promiseFocus(win);
+  let notifications = [...win.PanelUI.notificationPanel.children].filter(
+    n => !n.hidden
+  );
+  is(
+    notifications.length,
+    1,
+    "PanelUI doorhanger is only displaying one notification."
+  );
+  let doorhanger = notifications[0];
+  let button = doorhanger.secondaryButton;
+  button.click();
+
+  await BrowserTestUtils.closeWindow(win);
+  await SimpleTest.promiseFocus(window);
+
+  is(
+    PanelUI.notificationPanel.state,
+    "closed",
+    "The background window's doorhanger is closed."
+  );
+  is(
+    PanelUI.menuButton.hasAttribute("badge-status"),
+    true,
+    "The dismissed notification should still have a badge status"
+  );
+
+  AppMenuNotifications.removeNotification(/.*/);
+});
diff --git a/browser/components/customizableui/test/browser_panel_keyboard_navigation.js b/browser/components/customizableui/test/browser_panel_keyboard_navigation.js
new file mode 100644
index 0000000000..9b5ad48cce
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panel_keyboard_navigation.js
@@ -0,0 +1,326 @@
+"use strict";
+
+/**
+ * Test keyboard navigation in the app menu panel.
+ */
+
+const kHelpButtonId = "appMenu-help-button2";
+
+function getEnabledNavigableElementsForView(panelView) {
+  return Array.from(
+    panelView.querySelectorAll("button,toolbarbutton,menulist,.text-link")
+  ).filter(element => {
+    let bounds = element.getBoundingClientRect();
+    return !element.disabled && bounds.width > 0 && bounds.height > 0;
+  });
+}
+
+add_task(async function testUpDownKeys() {
+  await gCUITestUtils.openMainMenu();
+
+  let buttons = getEnabledNavigableElementsForView(PanelUI.mainView);
+
+  for (let button of buttons) {
+    if (button.disabled) {
+      continue;
+    }
+    EventUtils.synthesizeKey("KEY_ArrowDown");
+    Assert.equal(
+      document.commandDispatcher.focusedElement,
+      button,
+      "The correct button should be focused after navigating downward"
+    );
+  }
+
+  EventUtils.synthesizeKey("KEY_ArrowDown");
+  Assert.equal(
+    document.commandDispatcher.focusedElement,
+    buttons[0],
+    "Pressing upwards should cycle around and select the first button again"
+  );
+
+  for (let i = buttons.length - 1; i >= 0; --i) {
+    let button = buttons[i];
+    if (button.disabled) {
+      continue;
+    }
+    EventUtils.synthesizeKey("KEY_ArrowUp");
+    Assert.equal(
+      document.commandDispatcher.focusedElement,
+      button,
+      "The first button should be focused after navigating upward"
+    );
+  }
+
+  await gCUITestUtils.hideMainMenu();
+});
+
+add_task(async function testHomeEndKeys() {
+  await gCUITestUtils.openMainMenu();
+
+  let buttons = getEnabledNavigableElementsForView(PanelUI.mainView);
+  let enabledButtons = buttons.filter(btn => !btn.disabled);
+  let firstButton = enabledButtons[0];
+  let lastButton = enabledButtons.pop();
+
+  Assert.ok(firstButton != lastButton, "There is more than one button");
+
+  EventUtils.synthesizeKey("KEY_End");
+  Assert.equal(
+    document.commandDispatcher.focusedElement,
+    lastButton,
+    "The last button should be focused after pressing End"
+  );
+
+  EventUtils.synthesizeKey("KEY_Home");
+  Assert.equal(
+    document.commandDispatcher.focusedElement,
+    firstButton,
+    "The first button should be focused after pressing Home"
+  );
+
+  await gCUITestUtils.hideMainMenu();
+});
+
+add_task(async function testEnterKeyBehaviors() {
+  await gCUITestUtils.openMainMenu();
+
+  let buttons = getEnabledNavigableElementsForView(PanelUI.mainView);
+
+  // Navigate to the 'Help' button, which points to a subview.
+  EventUtils.synthesizeKey("KEY_ArrowUp");
+  let focusedElement = document.commandDispatcher.focusedElement;
+  Assert.equal(
+    focusedElement,
+    buttons[buttons.length - 1],
+    "The last button should be focused after navigating upward"
+  );
+
+  // Make sure the Help button is in focus.
+  while (
+    !focusedElement ||
+    !focusedElement.id ||
+    focusedElement.id != kHelpButtonId
+  ) {
+    EventUtils.synthesizeKey("KEY_ArrowUp");
+    focusedElement = document.commandDispatcher.focusedElement;
+  }
+  EventUtils.synthesizeKey("KEY_Enter");
+
+  let helpView = document.getElementById("PanelUI-helpView");
+  await BrowserTestUtils.waitForEvent(helpView, "ViewShown");
+
+  let helpButtons = getEnabledNavigableElementsForView(helpView);
+  Assert.ok(
+    helpButtons[0].classList.contains("subviewbutton-back"),
+    "First button in help view should be a back button"
+  );
+
+  // For posterity, check navigating the subview using up/ down arrow keys as well.
+  // When opening a subview, the first control *after* the Back button gets
+  // focus.
+  EventUtils.synthesizeKey("KEY_ArrowUp");
+  focusedElement = document.commandDispatcher.focusedElement;
+  Assert.equal(
+    focusedElement,
+    helpButtons[0],
+    "The Back button should be focused after navigating upward"
+  );
+  for (let i = helpButtons.length - 1; i >= 0; --i) {
+    let button = helpButtons[i];
+    if (button.disabled) {
+      continue;
+    }
+    EventUtils.synthesizeKey("KEY_ArrowUp");
+    focusedElement = document.commandDispatcher.focusedElement;
+    Assert.equal(
+      focusedElement,
+      button,
+      "The previous button should be focused after navigating upward"
+    );
+  }
+
+  // Make sure the back button is in focus again.
+  while (focusedElement != helpButtons[0]) {
+    EventUtils.synthesizeKey("KEY_ArrowDown");
+    focusedElement = document.commandDispatcher.focusedElement;
+  }
+
+  // The first button is the back button. Hittin Enter should navigate us back.
+  let promise = BrowserTestUtils.waitForEvent(PanelUI.mainView, "ViewShown");
+  EventUtils.synthesizeKey("KEY_Enter");
+  await promise;
+
+  // Let's test a 'normal' command button.
+  focusedElement = document.commandDispatcher.focusedElement;
+  const kFindButtonId = "appMenu-find-button2";
+  while (
+    !focusedElement ||
+    !focusedElement.id ||
+    focusedElement.id != kFindButtonId
+  ) {
+    EventUtils.synthesizeKey("KEY_ArrowUp");
+    focusedElement = document.commandDispatcher.focusedElement;
+  }
+  let findBarPromise = gBrowser.isFindBarInitialized()
+    ? null
+    : BrowserTestUtils.waitForEvent(gBrowser.selectedTab, "TabFindInitialized");
+  Assert.equal(
+    focusedElement.id,
+    kFindButtonId,
+    "Find button should be selected"
+  );
+
+  await gCUITestUtils.hidePanelMultiView(PanelUI.panel, () =>
+    EventUtils.synthesizeKey("KEY_Enter")
+  );
+
+  await findBarPromise;
+  Assert.ok(!gFindBar.hidden, "Findbar should have opened");
+  gFindBar.close();
+});
+
+add_task(async function testLeftRightKeys() {
+  await gCUITestUtils.openMainMenu();
+
+  // Navigate to the 'Help' button, which points to a subview.
+  let focusedElement = document.commandDispatcher.focusedElement;
+  while (
+    !focusedElement ||
+    !focusedElement.id ||
+    focusedElement.id != kHelpButtonId
+  ) {
+    EventUtils.synthesizeKey("KEY_ArrowUp");
+    focusedElement = document.commandDispatcher.focusedElement;
+  }
+  Assert.equal(
+    focusedElement.id,
+    kHelpButtonId,
+    "The last button should be focused after navigating upward"
+  );
+
+  // Hitting ArrowRight on a button that points to a subview should navigate us
+  // there.
+  EventUtils.synthesizeKey("KEY_ArrowRight");
+  let helpView = document.getElementById("PanelUI-helpView");
+  await BrowserTestUtils.waitForEvent(helpView, "ViewShown");
+
+  // Hitting ArrowLeft should navigate us back.
+  let promise = BrowserTestUtils.waitForEvent(PanelUI.mainView, "ViewShown");
+  EventUtils.synthesizeKey("KEY_ArrowLeft");
+  await promise;
+
+  focusedElement = document.commandDispatcher.focusedElement;
+  Assert.equal(
+    focusedElement.id,
+    kHelpButtonId,
+    "Help button should be focused again now that we're back in the main view"
+  );
+
+  await gCUITestUtils.hideMainMenu();
+});
+
+add_task(async function testTabKey() {
+  await gCUITestUtils.openMainMenu();
+
+  let buttons = getEnabledNavigableElementsForView(PanelUI.mainView);
+
+  for (let button of buttons) {
+    if (button.disabled) {
+      continue;
+    }
+    EventUtils.synthesizeKey("KEY_Tab");
+    Assert.equal(
+      document.commandDispatcher.focusedElement,
+      button,
+      "The correct button should be focused after tabbing"
+    );
+  }
+
+  EventUtils.synthesizeKey("KEY_Tab");
+  Assert.equal(
+    document.commandDispatcher.focusedElement,
+    buttons[0],
+    "Pressing tab should cycle around and select the first button again"
+  );
+
+  for (let i = buttons.length - 1; i >= 0; --i) {
+    let button = buttons[i];
+    if (button.disabled) {
+      continue;
+    }
+    EventUtils.synthesizeKey("KEY_Tab", { shiftKey: true });
+    Assert.equal(
+      document.commandDispatcher.focusedElement,
+      button,
+      "The correct button should be focused after shift + tabbing"
+    );
+  }
+
+  EventUtils.synthesizeKey("KEY_Tab", { shiftKey: true });
+  Assert.equal(
+    document.commandDispatcher.focusedElement,
+    buttons[buttons.length - 1],
+    "Pressing shift + tab should cycle around and select the last button again"
+  );
+
+  await gCUITestUtils.hideMainMenu();
+});
+
+add_task(async function testInterleavedTabAndArrowKeys() {
+  await gCUITestUtils.openMainMenu();
+
+  let buttons = getEnabledNavigableElementsForView(PanelUI.mainView);
+  let tab = false;
+
+  for (let button of buttons) {
+    if (button.disabled) {
+      continue;
+    }
+    if (tab) {
+      EventUtils.synthesizeKey("KEY_Tab");
+    } else {
+      EventUtils.synthesizeKey("KEY_ArrowDown");
+    }
+    tab = !tab;
+  }
+
+  Assert.equal(
+    document.commandDispatcher.focusedElement,
+    buttons[buttons.length - 1],
+    "The last button should be focused after a mix of Tab and ArrowDown"
+  );
+
+  await gCUITestUtils.hideMainMenu();
+});
+
+add_task(async function testSpaceDownAfterTabNavigation() {
+  await gCUITestUtils.openMainMenu();
+
+  let buttons = getEnabledNavigableElementsForView(PanelUI.mainView);
+  let button;
+
+  for (button of buttons) {
+    if (button.disabled) {
+      continue;
+    }
+    EventUtils.synthesizeKey("KEY_Tab");
+    if (button.id == kHelpButtonId) {
+      break;
+    }
+  }
+
+  Assert.equal(
+    document.commandDispatcher.focusedElement,
+    button,
+    "Help button should be focused after tabbing to it."
+  );
+
+  // Pressing down space on a button that points to a subview should navigate us
+  // there, before keyup.
+  EventUtils.synthesizeKey(" ", { type: "keydown" });
+  let helpView = document.getElementById("PanelUI-helpView");
+  await BrowserTestUtils.waitForEvent(helpView, "ViewShown");
+
+  await gCUITestUtils.hideMainMenu();
+});
diff --git a/browser/components/customizableui/test/browser_panel_locationSpecific.js b/browser/components/customizableui/test/browser_panel_locationSpecific.js
new file mode 100644
index 0000000000..1668751d9f
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panel_locationSpecific.js
@@ -0,0 +1,78 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/*
+ * This test creates multiple panels, one that has been tagged as location specific
+ * and one that isn't. When the location changes, the specific panel should close.
+ * The non-specific panel should remain open.
+ *
+ */
+
+add_task(async function () {
+  let specificPanel = document.createXULElement("panel");
+  specificPanel.setAttribute("locationspecific", "true");
+  specificPanel.setAttribute("noautohide", "true");
+  specificPanel.style.height = "100px";
+  specificPanel.style.width = "100px";
+
+  let generalPanel = document.createXULElement("panel");
+  generalPanel.setAttribute("noautohide", "true");
+  generalPanel.style.height = "100px";
+  generalPanel.style.width = "100px";
+
+  let anchor = document.getElementById(CustomizableUI.AREA_NAVBAR);
+
+  anchor.appendChild(specificPanel);
+  anchor.appendChild(generalPanel);
+  is(specificPanel.state, "closed", "specificPanel starts as closed");
+  is(generalPanel.state, "closed", "generalPanel starts as closed");
+
+  let specificPanelPromise = BrowserTestUtils.waitForEvent(
+    specificPanel,
+    "popupshown"
+  );
+
+  specificPanel.openPopupAtScreen(0, 0);
+
+  await specificPanelPromise;
+  is(specificPanel.state, "open", "specificPanel has been opened");
+
+  let generalPanelPromise = BrowserTestUtils.waitForEvent(
+    generalPanel,
+    "popupshown"
+  );
+
+  generalPanel.openPopupAtScreen(100, 0);
+
+  await generalPanelPromise;
+  is(generalPanel.state, "open", "generalPanel has been opened");
+
+  let specificPanelHiddenPromise = BrowserTestUtils.waitForEvent(
+    specificPanel,
+    "popuphidden"
+  );
+
+  // Simulate a location change, and check which panel closes.
+  let browser = gBrowser.selectedBrowser;
+  let loaded = BrowserTestUtils.browserLoaded(browser);
+  BrowserTestUtils.loadURIString(browser, "http://mochi.test:8888/#0");
+  await loaded;
+
+  await specificPanelHiddenPromise;
+
+  is(
+    specificPanel.state,
+    "closed",
+    "specificPanel panel is closed after location change"
+  );
+  is(
+    generalPanel.state,
+    "open",
+    "generalPanel is still open after location change"
+  );
+
+  specificPanel.remove();
+  generalPanel.remove();
+});
diff --git a/browser/components/customizableui/test/browser_panel_toggle.js b/browser/components/customizableui/test/browser_panel_toggle.js
new file mode 100644
index 0000000000..cba441e8e5
--- /dev/null
+++ b/browser/components/customizableui/test/browser_panel_toggle.js
@@ -0,0 +1,53 @@
+/* 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";
+
+/**
+ * Test opening and closing the menu panel UI.
+ */
+
+// Show and hide the menu panel programmatically without an event (like UITour.sys.mjs would)
+add_task(async function () {
+  await gCUITestUtils.openMainMenu();
+
+  is(
+    PanelUI.panel.getAttribute("panelopen"),
+    "true",
+    "Check that panel has panelopen attribute"
+  );
+  is(PanelUI.panel.state, "open", "Check that panel state is 'open'");
+
+  await gCUITestUtils.hideMainMenu();
+
+  ok(
+    !PanelUI.panel.hasAttribute("panelopen"),
+    "Check that panel doesn't have the panelopen attribute"
+  );
+  is(PanelUI.panel.state, "closed", "Check that panel state is 'closed'");
+});
+
+// Toggle the menu panel open and closed
+add_task(async function () {
+  await gCUITestUtils.openPanelMultiView(PanelUI.panel, PanelUI.mainView, () =>
+    PanelUI.toggle({ type: "command" })
+  );
+
+  is(
+    PanelUI.panel.getAttribute("panelopen"),
+    "true",
+    "Check that panel has panelopen attribute"
+  );
+  is(PanelUI.panel.state, "open", "Check that panel state is 'open'");
+
+  await gCUITestUtils.hidePanelMultiView(PanelUI.panel, () =>
+    PanelUI.toggle({ type: "command" })
+  );
+
+  ok(
+    !PanelUI.panel.hasAttribute("panelopen"),
+    "Check that panel doesn't have the panelopen attribute"
+  );
+  is(PanelUI.panel.state, "closed", "Check that panel state is 'closed'");
+});
diff --git a/browser/components/customizableui/test/browser_proton_moreTools_panel.js b/browser/components/customizableui/test/browser_proton_moreTools_panel.js
new file mode 100644
index 0000000000..d78cf946f8
--- /dev/null
+++ b/browser/components/customizableui/test/browser_proton_moreTools_panel.js
@@ -0,0 +1,54 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+XPCOMUtils.defineLazyGetter(this, "DevToolsStartup", () => {
+  return Cc["@mozilla.org/devtools/startup-clh;1"].getService(
+    Ci.nsICommandLineHandler
+  ).wrappedJSObject;
+});
+
+// Test activating the developer button shows the More Tools panel.
+add_task(async function testDevToolsPanelInToolbar() {
+  // We need to force DevToolsStartup to rebuild the developer tool toggle so that
+  // proton prefs are applied to the new browser window for this test.
+  DevToolsStartup.developerToggleCreated = false;
+  CustomizableUI.destroyWidget("developer-button");
+
+  const win = await BrowserTestUtils.openNewBrowserWindow();
+
+  CustomizableUI.addWidgetToArea(
+    "developer-button",
+    CustomizableUI.AREA_NAVBAR
+  );
+
+  // Test the developer tools panel is showing.
+  let button = document.getElementById("developer-button");
+  let devToolsView = PanelMultiView.getViewNode(
+    document,
+    "PanelUI-developer-tools"
+  );
+  let devToolsShownPromise = BrowserTestUtils.waitForEvent(
+    devToolsView,
+    "ViewShown"
+  );
+
+  EventUtils.synthesizeMouseAtCenter(button, {});
+  await devToolsShownPromise;
+  ok(true, "Dev Tools view is showing");
+  is(
+    devToolsView.children.length,
+    1,
+    "Dev tools subview is the only child of panel"
+  );
+  is(
+    devToolsView.children[0].id,
+    "PanelUI-developer-tools-view",
+    "Dev tools child has correct id"
+  );
+
+  // Cleanup
+  await BrowserTestUtils.closeWindow(win);
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/browser_proton_toolbar_hide_toolbarbuttons.js b/browser/components/customizableui/test/browser_proton_toolbar_hide_toolbarbuttons.js
new file mode 100644
index 0000000000..ef0a44d6b1
--- /dev/null
+++ b/browser/components/customizableui/test/browser_proton_toolbar_hide_toolbarbuttons.js
@@ -0,0 +1,287 @@
+/* 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";
+
+ChromeUtils.defineModuleGetter(
+  this,
+  "HomePage",
+  "resource:///modules/HomePage.jsm"
+);
+
+const kPrefProtonToolbarVersion = "browser.proton.toolbar.version";
+const kPrefHomeButtonUsed = "browser.engagement.home-button.has-used";
+const kPrefLibraryButtonUsed = "browser.engagement.library-button.has-used";
+const kPrefSidebarButtonUsed = "browser.engagement.sidebar-button.has-used";
+
+async function testToolbarButtons(aActions) {
+  let {
+    shouldRemoveHomeButton,
+    shouldRemoveLibraryButton,
+    shouldRemoveSidebarButton,
+    shouldUpdateVersion,
+  } = aActions;
+  const defaultPlacements = [
+    "back-button",
+    "forward-button",
+    "stop-reload-button",
+    "home-button",
+    "customizableui-special-spring1",
+    "urlbar-container",
+    "customizableui-special-spring2",
+    "downloads-button",
+    "library-button",
+    "sidebar-button",
+    "fxa-toolbar-menu-button",
+  ];
+  let oldState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+
+  Assert.equal(
+    Services.prefs.getIntPref(kPrefProtonToolbarVersion),
+    0,
+    "Toolbar proton version is 0"
+  );
+
+  let CustomizableUIInternal = CustomizableUI.getTestOnlyInternalProp(
+    "CustomizableUIInternal"
+  );
+
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", {
+    placements: {
+      "nav-bar": defaultPlacements,
+    },
+  });
+  CustomizableUIInternal._updateForNewProtonVersion();
+
+  let navbarPlacements =
+    CustomizableUI.getTestOnlyInternalProp("gSavedState").placements["nav-bar"];
+  let includesHomeButton = navbarPlacements.includes("home-button");
+  let includesLibraryButton = navbarPlacements.includes("library-button");
+  let includesSidebarButton = navbarPlacements.includes("sidebar-button");
+
+  Assert.equal(
+    !includesHomeButton,
+    shouldRemoveHomeButton,
+    "Correctly handles home button"
+  );
+  Assert.equal(
+    !includesLibraryButton,
+    shouldRemoveLibraryButton,
+    "Correctly handles library button"
+  );
+  Assert.equal(
+    !includesSidebarButton,
+    shouldRemoveSidebarButton,
+    "Correctly handles sidebar button"
+  );
+
+  let toolbarVersion = Services.prefs.getIntPref(kPrefProtonToolbarVersion);
+  if (shouldUpdateVersion) {
+    Assert.ok(toolbarVersion >= 1, "Toolbar proton version updated");
+  } else {
+    Assert.ok(toolbarVersion == 0, "Toolbar proton version not updated");
+  }
+
+  // Cleanup
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", oldState);
+}
+
+/**
+ * Checks that the home button is removed from the nav-bar under
+ * these conditions: proton must be enabled, the toolbar engagement
+ * pref is false, and the homepage is about:home or about:blank.
+ * Otherwise, the home button should remain if it was previously
+ * in the navbar.
+ * Also checks that the library button is removed from the nav-bar
+ * if proton is enabled and the toolbar engagement pref is false.
+ */
+add_task(async function testButtonRemoval() {
+  // Ensure the engagement prefs are set to their default values
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      [kPrefHomeButtonUsed, false],
+      [kPrefLibraryButtonUsed, false],
+      [kPrefSidebarButtonUsed, false],
+    ],
+  });
+
+  let tests = [
+    // Proton enabled without home and library engagement
+    {
+      prefs: [],
+      actions: {
+        shouldRemoveHomeButton: true,
+        shouldRemoveLibraryButton: true,
+        shouldRemoveSidebarButton: true,
+        shouldUpdateVersion: true,
+      },
+    },
+    // Proton enabled with home engagement
+    {
+      prefs: [[kPrefHomeButtonUsed, true]],
+      actions: {
+        shouldRemoveHomeButton: false,
+        shouldRemoveLibraryButton: true,
+        shouldRemoveSidebarButton: true,
+        shouldUpdateVersion: true,
+      },
+    },
+    // Proton enabled with custom homepage
+    {
+      prefs: [],
+      actions: {
+        shouldRemoveHomeButton: false,
+        shouldRemoveLibraryButton: true,
+        shouldRemoveSidebarButton: true,
+        shouldUpdateVersion: true,
+      },
+      async fn() {
+        HomePage.safeSet("https://example.com");
+      },
+    },
+    // Proton enabled with library engagement
+    {
+      prefs: [[kPrefLibraryButtonUsed, true]],
+      actions: {
+        shouldRemoveHomeButton: true,
+        shouldRemoveLibraryButton: false,
+        shouldRemoveSidebarButton: true,
+        shouldUpdateVersion: true,
+      },
+    },
+    // Proton enabled with sidebar engagement
+    {
+      prefs: [[kPrefSidebarButtonUsed, true]],
+      actions: {
+        shouldRemoveHomeButton: true,
+        shouldRemoveLibraryButton: true,
+        shouldRemoveSidebarButton: false,
+        shouldUpdateVersion: true,
+      },
+    },
+  ];
+
+  for (let test of tests) {
+    await SpecialPowers.pushPrefEnv({
+      set: [[kPrefProtonToolbarVersion, 0], ...test.prefs],
+    });
+    if (test.fn) {
+      await test.fn();
+    }
+    testToolbarButtons(test.actions);
+    HomePage.reset();
+    await SpecialPowers.popPrefEnv();
+  }
+});
+
+/**
+ * Checks that a null saved state (new profile) does not prevent migration.
+ */
+add_task(async function testNullSavedState() {
+  await SpecialPowers.pushPrefEnv({
+    set: [[kPrefProtonToolbarVersion, 0]],
+  });
+  let oldState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+
+  Assert.equal(
+    Services.prefs.getIntPref(kPrefProtonToolbarVersion),
+    0,
+    "Toolbar proton version is 0"
+  );
+
+  let CustomizableUIInternal = CustomizableUI.getTestOnlyInternalProp(
+    "CustomizableUIInternal"
+  );
+  CustomizableUIInternal.initialize();
+
+  Assert.ok(
+    Services.prefs.getIntPref(kPrefProtonToolbarVersion) >= 1,
+    "Toolbar proton version updated"
+  );
+  let navbarPlacements = CustomizableUI.getTestOnlyInternalProp("gAreas")
+    .get("nav-bar")
+    .get("defaultPlacements");
+  Assert.ok(
+    !navbarPlacements.includes("home-button"),
+    "Home button isn't included by default"
+  );
+  Assert.ok(
+    !navbarPlacements.includes("library-button"),
+    "Library button isn't included by default"
+  );
+  Assert.ok(
+    !navbarPlacements.includes("sidebar-button"),
+    "Sidebar button isn't included by default"
+  );
+
+  // Cleanup
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", oldState);
+  await SpecialPowers.popPrefEnv();
+  // Re-initialize to prevent future test failures
+  CustomizableUIInternal.initialize();
+});
+
+/**
+ * Checks that a saved state that is missing nav-bar placements does not prevent migration.
+ */
+add_task(async function testNoNavbarPlacements() {
+  await SpecialPowers.pushPrefEnv({
+    set: [[kPrefProtonToolbarVersion, 0]],
+  });
+
+  let oldState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+
+  Assert.equal(
+    Services.prefs.getIntPref(kPrefProtonToolbarVersion),
+    0,
+    "Toolbar proton version is 0"
+  );
+
+  let CustomizableUIInternal = CustomizableUI.getTestOnlyInternalProp(
+    "CustomizableUIInternal"
+  );
+
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", {
+    placements: { "widget-overflow-fixed-list": [] },
+  });
+  CustomizableUIInternal._updateForNewProtonVersion();
+
+  Assert.ok(true, "_updateForNewProtonVersion didn't throw");
+
+  // Cleanup
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", oldState);
+
+  await SpecialPowers.popPrefEnv();
+});
+
+/**
+ * Checks that a saved state that is missing the placements value does not prevent migration.
+ */
+add_task(async function testNullPlacements() {
+  await SpecialPowers.pushPrefEnv({
+    set: [[kPrefProtonToolbarVersion, 0]],
+  });
+
+  let oldState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+
+  Assert.equal(
+    Services.prefs.getIntPref(kPrefProtonToolbarVersion),
+    0,
+    "Toolbar proton version is 0"
+  );
+
+  let CustomizableUIInternal = CustomizableUI.getTestOnlyInternalProp(
+    "CustomizableUIInternal"
+  );
+
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", {});
+  CustomizableUIInternal._updateForNewProtonVersion();
+
+  Assert.ok(true, "_updateForNewProtonVersion didn't throw");
+
+  // Cleanup
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", oldState);
+
+  await SpecialPowers.popPrefEnv();
+});
diff --git a/browser/components/customizableui/test/browser_registerArea.js b/browser/components/customizableui/test/browser_registerArea.js
new file mode 100644
index 0000000000..2900c9eb8b
--- /dev/null
+++ b/browser/components/customizableui/test/browser_registerArea.js
@@ -0,0 +1,28 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Tests that a toolbar area can be registered with overflowable: false
+ * as one of its properties, and this results in a non-overflowable
+ * toolbar.
+ */
+add_task(async function test_overflowable_false() {
+  registerCleanupFunction(removeCustomToolbars);
+
+  const kToolbarId = "no-overflow-toolbar";
+  createToolbarWithPlacements(kToolbarId, ["spring"], {
+    overflowable: false,
+  });
+
+  let node = CustomizableUI.getWidget(kToolbarId).forWindow(window).node;
+  Assert.ok(
+    !node.hasAttribute("overflowable"),
+    "Toolbar should not be overflowable"
+  );
+  Assert.ok(
+    !node.overflowable,
+    "OverflowableToolbar instance should not have been created."
+  );
+});
diff --git a/browser/components/customizableui/test/browser_reload_tab.js b/browser/components/customizableui/test/browser_reload_tab.js
new file mode 100644
index 0000000000..b12f5ee0e9
--- /dev/null
+++ b/browser/components/customizableui/test/browser_reload_tab.js
@@ -0,0 +1,99 @@
+"use strict";
+
+/**
+ * Check that customize mode doesn't break when its tab is reloaded.
+ */
+add_task(async function reload_tab() {
+  let initialTab = gBrowser.selectedTab;
+  let customizeTab = BrowserTestUtils.addTab(gBrowser, "about:blank");
+  gCustomizeMode.setTab(customizeTab);
+  let customizationContainer = document.getElementById(
+    "customization-container"
+  );
+
+  is(
+    customizationContainer.clientWidth,
+    0,
+    "Customization container shouldn't be visible (X)"
+  );
+  is(
+    customizationContainer.clientHeight,
+    0,
+    "Customization container shouldn't be visible (Y)"
+  );
+
+  let customizePromise = BrowserTestUtils.waitForEvent(
+    gNavToolbox,
+    "customizationready"
+  );
+  gCustomizeMode.enter();
+  await customizePromise;
+
+  let tabReloaded = new Promise(resolve => {
+    gBrowser.addTabsProgressListener({
+      async onLocationChange(aBrowser) {
+        if (customizeTab.linkedBrowser == aBrowser) {
+          gBrowser.removeTabsProgressListener(this);
+          await Promise.resolve();
+          resolve();
+        }
+      },
+    });
+  });
+  gBrowser.reloadTab(customizeTab);
+  await tabReloaded;
+
+  is(
+    gBrowser.getIcon(customizeTab),
+    "chrome://browser/skin/customize.svg",
+    "Tab should have customize icon"
+  );
+  is(
+    customizeTab.getAttribute("customizemode"),
+    "true",
+    "Tab should be in customize mode"
+  );
+  ok(
+    customizationContainer.clientWidth > 0,
+    "Customization container should be visible (X)"
+  );
+  ok(
+    customizationContainer.clientHeight > 0,
+    "Customization container should be visible (Y)"
+  );
+
+  customizePromise = BrowserTestUtils.waitForEvent(
+    gNavToolbox,
+    "aftercustomization"
+  );
+  await BrowserTestUtils.switchTab(gBrowser, initialTab);
+  await customizePromise;
+
+  customizePromise = BrowserTestUtils.waitForEvent(
+    gNavToolbox,
+    "customizationready"
+  );
+  await BrowserTestUtils.switchTab(gBrowser, customizeTab);
+  await customizePromise;
+
+  is(
+    gBrowser.getIcon(customizeTab),
+    "chrome://browser/skin/customize.svg",
+    "Tab should still have customize icon"
+  );
+  is(
+    customizeTab.getAttribute("customizemode"),
+    "true",
+    "Tab should still be in customize mode"
+  );
+  ok(
+    customizationContainer.clientWidth > 0,
+    "Customization container should still be visible (X)"
+  );
+  ok(
+    customizationContainer.clientHeight > 0,
+    "Customization container should still be visible (Y)"
+  );
+
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_remote_attribute.js b/browser/components/customizableui/test/browser_remote_attribute.js
new file mode 100644
index 0000000000..543e62e2bc
--- /dev/null
+++ b/browser/components/customizableui/test/browser_remote_attribute.js
@@ -0,0 +1,73 @@
+/* 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";
+
+/**
+ * These tests check that the remote attribute is true for remote panels.
+ * This attribute is needed for Mac to properly render the panel.
+ */
+add_task(async function check_remote_attribute() {
+  // The panel is created on the fly, so we can't simply wait for focus
+  // inside it.
+  //let pocketPanelShown = BrowserTestUtils.waitForEvent(
+  //  document,
+  //  "popupshown",
+  //  true
+  //);
+  let pocketPanelShown = popupShown(document);
+  // Using Pocket panel as it's an available remote panel.
+  let pocketButton = document.getElementById("save-to-pocket-button");
+  pocketButton.click();
+  await pocketPanelShown;
+
+  let pocketPanel = document.getElementById("customizationui-widget-panel");
+  is(
+    pocketPanel.getAttribute("remote"),
+    "true",
+    "Pocket panel has remote attribute"
+  );
+
+  // Close panel and cleanup.
+  let pocketPanelHidden = popupHidden(pocketPanel);
+  pocketPanel.hidePopup();
+  await pocketPanelHidden;
+});
+
+add_task(async function check_remote_attribute_overflow() {
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  let overflowPanel = win.document.getElementById("widget-overflow");
+  overflowPanel.setAttribute("animate", "false");
+
+  // Force a narrow window to get an overflow toolbar.
+  win.resizeTo(kForceOverflowWidthPx, win.outerHeight);
+  let navbar = win.document.getElementById(CustomizableUI.AREA_NAVBAR);
+  await TestUtils.waitForCondition(() => navbar.hasAttribute("overflowing"));
+
+  // Open the overflow panel view.
+  let overflowPanelShown = popupShown(overflowPanel);
+  let overflowPanelButton = win.document.getElementById(
+    "nav-bar-overflow-button"
+  );
+  overflowPanelButton.click();
+  await overflowPanelShown;
+
+  // Using Pocket panel as it's an available remote panel.
+  let pocketButton = win.document.getElementById("save-to-pocket-button");
+  pocketButton.click();
+  await BrowserTestUtils.waitForEvent(win.document, "ViewShown");
+
+  is(
+    overflowPanel.getAttribute("remote"),
+    "true",
+    "Pocket overflow panel has remote attribute"
+  );
+
+  // Close panel and cleanup.
+  let overflowPanelHidden = popupHidden(overflowPanel);
+  overflowPanel.hidePopup();
+  await overflowPanelHidden;
+  overflowPanel.removeAttribute("animate");
+  await BrowserTestUtils.closeWindow(win);
+});
diff --git a/browser/components/customizableui/test/browser_remote_tabs_button.js b/browser/components/customizableui/test/browser_remote_tabs_button.js
new file mode 100644
index 0000000000..094335d4b1
--- /dev/null
+++ b/browser/components/customizableui/test/browser_remote_tabs_button.js
@@ -0,0 +1,100 @@
+/* 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";
+
+let { Service } = ChromeUtils.importESModule(
+  "resource://services-sync/service.sys.mjs"
+);
+const { UIState } = ChromeUtils.importESModule(
+  "resource://services-sync/UIState.sys.mjs"
+);
+
+let getState;
+let originalSync;
+let syncWasCalled = false;
+
+// TODO: This test should probably be re-written, we don't really test much here.
+add_task(async function testSyncRemoteTabsButtonFunctionality() {
+  info("Test the Sync Remote Tabs button in the panel");
+  storeInitialValues();
+  mockFunctions();
+
+  // Force UI update.
+  Services.obs.notifyObservers(null, UIState.ON_UPDATE);
+
+  // add the sync remote tabs button to the panel
+  CustomizableUI.addWidgetToArea(
+    "sync-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  // check the button's functionality
+  await document.getElementById("nav-bar").overflowable.show();
+  info("The panel menu was opened");
+
+  let syncRemoteTabsBtn = document.getElementById("sync-button");
+  ok(
+    syncRemoteTabsBtn,
+    "The sync remote tabs button was added to the Panel Menu"
+  );
+  // click the button - the panel should open.
+  syncRemoteTabsBtn.click();
+  let remoteTabsPanel = document.getElementById("PanelUI-remotetabs");
+  let viewShown = BrowserTestUtils.waitForEvent(remoteTabsPanel, "ViewShown");
+  await viewShown;
+  ok(remoteTabsPanel.getAttribute("visible"), "Sync Panel is in view");
+
+  // Find and click the "setup" button.
+  let syncNowButton = document.getElementById("PanelUI-remotetabs-syncnow");
+  syncNowButton.click();
+  info("The sync now button was clicked");
+
+  await TestUtils.waitForCondition(() => syncWasCalled);
+
+  // We need to stop the Syncing animation manually otherwise the button
+  // will be disabled at the beginning of a next test.
+  gSync._onActivityStop();
+});
+
+add_task(async function asyncCleanup() {
+  // reset the panel UI to the default state
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "The panel UI is in default state again.");
+
+  if (isOverflowOpen()) {
+    let panelHidePromise = promiseOverflowHidden(window);
+    PanelUI.overflowPanel.hidePopup();
+    await panelHidePromise;
+  }
+
+  restoreValues();
+});
+
+function mockFunctions() {
+  // mock UIState.get()
+  UIState.get = () => ({
+    status: UIState.STATUS_SIGNED_IN,
+    lastSync: new Date(),
+    email: "user@mozilla.com",
+  });
+
+  Service.sync = mocked_sync;
+}
+
+function mocked_sync() {
+  syncWasCalled = true;
+}
+
+function restoreValues() {
+  UIState.get = getState;
+  Service.sync = originalSync;
+}
+
+function storeInitialValues() {
+  getState = UIState.get;
+  originalSync = Service.sync;
+}
diff --git a/browser/components/customizableui/test/browser_remove_customized_specials.js b/browser/components/customizableui/test/browser_remove_customized_specials.js
new file mode 100644
index 0000000000..1f123d10cb
--- /dev/null
+++ b/browser/components/customizableui/test/browser_remove_customized_specials.js
@@ -0,0 +1,35 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Check that after a reset, we can still drag special nodes in customize mode
+ */
+add_task(async function () {
+  await startCustomizing();
+  CustomizableUI.addWidgetToArea("spring", "nav-bar", 5);
+  await gCustomizeMode.reset();
+  let springs = document.querySelectorAll("#nav-bar toolbarspring");
+  let lastSpring = springs[springs.length - 1];
+  let expectedPlacements = CustomizableUI.getWidgetIdsInArea("nav-bar");
+  info("Placements before drag: " + expectedPlacements.join(","));
+  let lastItem = document.getElementById(
+    expectedPlacements[expectedPlacements.length - 1]
+  );
+  await waitForElementShown(lastItem);
+  simulateItemDrag(lastSpring, lastItem, "end");
+  expectedPlacements.splice(expectedPlacements.indexOf(lastSpring.id), 1);
+  expectedPlacements.push(lastSpring.id);
+  let actualPlacements = CustomizableUI.getWidgetIdsInArea("nav-bar");
+  // Log these separately because Assert.deepEqual truncates the stringified versions...
+  info("Actual placements: " + actualPlacements.join(","));
+  info("Expected placements: " + expectedPlacements.join(","));
+  Assert.deepEqual(
+    expectedPlacements,
+    actualPlacements,
+    "Should be able to move spring"
+  );
+  await gCustomizeMode.reset();
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_reset_builtin_widget_currentArea.js b/browser/components/customizableui/test/browser_reset_builtin_widget_currentArea.js
new file mode 100644
index 0000000000..fa9e497734
--- /dev/null
+++ b/browser/components/customizableui/test/browser_reset_builtin_widget_currentArea.js
@@ -0,0 +1,26 @@
+/* Any copyright is dedicated to the Public Domain.
+ * https://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// Check that if we move a non-default, but builtin, widget to another area,
+// and then reset things, the currentArea is updated correctly.
+add_task(async function reset_should_not_keep_currentArea() {
+  CustomizableUI.addWidgetToArea(
+    "save-page-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+  // We can't check currentArea directly; check areaType which is based on it:
+  is(
+    CustomizableUI.getWidget("save-page-button").areaType,
+    CustomizableUI.TYPE_PANEL,
+    "Button should know it's in the overflow panel"
+  );
+  CustomizableUI.reset();
+  ok(
+    !CustomizableUI.getWidget("save-page-button").areaType,
+    "Button should know it's not in the overflow panel anymore"
+  );
+});
+
+registerCleanupFunction(() => CustomizableUI.reset());
diff --git a/browser/components/customizableui/test/browser_reset_dom_events.js b/browser/components/customizableui/test/browser_reset_dom_events.js
new file mode 100644
index 0000000000..2922fe481d
--- /dev/null
+++ b/browser/components/customizableui/test/browser_reset_dom_events.js
@@ -0,0 +1,34 @@
+"use strict";
+
+const widgetId = "import-button";
+const listener = {
+  _beforeCount: 0,
+  _afterCount: 0,
+  onWidgetBeforeDOMChange(node) {
+    if (node.id == widgetId) {
+      this._beforeCount++;
+    }
+  },
+  onWidgetAfterDOMChange(node) {
+    if (node.id == widgetId) {
+      this._afterCount++;
+    }
+  },
+};
+
+add_task(async function test_reset_dom_events() {
+  await startCustomizing();
+
+  CustomizableUI.addWidgetToArea(widgetId, CustomizableUI.AREA_BOOKMARKS);
+  CustomizableUI.addListener(listener);
+
+  info("Resetting");
+  await gCustomizeMode.reset();
+
+  is(listener._beforeCount, 1, "Should've been notified of the mutation");
+  is(listener._afterCount, 1, "Should've been notified of the mutation");
+
+  CustomizableUI.removeListener(listener);
+
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_screenshot_button_disabled.js b/browser/components/customizableui/test/browser_screenshot_button_disabled.js
new file mode 100644
index 0000000000..b8eca2b3d3
--- /dev/null
+++ b/browser/components/customizableui/test/browser_screenshot_button_disabled.js
@@ -0,0 +1,22 @@
+/* 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 testScreenshotButtonPrefDisabled() {
+  info("Test the Screenshots widget not available");
+
+  Assert.ok(
+    Services.prefs.getBoolPref("extensions.screenshots.disabled"),
+    "Sceenshots feature is disabled"
+  );
+
+  CustomizableUI.addWidgetToArea(
+    "screenshot-button",
+    CustomizableUI.AREA_NAVBAR
+  );
+
+  let screenshotBtn = document.getElementById("screenshot-button");
+  Assert.ok(!screenshotBtn, "Screenshot button is unavailable");
+});
diff --git a/browser/components/customizableui/test/browser_sidebar_toggle.js b/browser/components/customizableui/test/browser_sidebar_toggle.js
new file mode 100644
index 0000000000..5742f368ee
--- /dev/null
+++ b/browser/components/customizableui/test/browser_sidebar_toggle.js
@@ -0,0 +1,58 @@
+/* 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";
+
+registerCleanupFunction(async function () {
+  await resetCustomization();
+
+  // Ensure sidebar is hidden after each test:
+  if (!document.getElementById("sidebar-box").hidden) {
+    SidebarUI.hide();
+  }
+});
+
+var showSidebar = async function (win = window) {
+  let button = win.document.getElementById("sidebar-button");
+  let sidebarFocusedPromise = BrowserTestUtils.waitForEvent(
+    win.document,
+    "SidebarFocused"
+  );
+  EventUtils.synthesizeMouseAtCenter(button, {}, win);
+  await sidebarFocusedPromise;
+  ok(win.SidebarUI.isOpen, "Sidebar is opened");
+  ok(button.hasAttribute("checked"), "Toolbar button is checked");
+};
+
+var hideSidebar = async function (win = window) {
+  let button = win.document.getElementById("sidebar-button");
+  EventUtils.synthesizeMouseAtCenter(button, {}, win);
+  ok(!win.SidebarUI.isOpen, "Sidebar is closed");
+  ok(!button.hasAttribute("checked"), "Toolbar button isn't checked");
+};
+
+// Check the sidebar widget shows the default items
+add_task(async function () {
+  CustomizableUI.addWidgetToArea("sidebar-button", "nav-bar");
+
+  await showSidebar();
+  is(SidebarUI.currentID, "viewBookmarksSidebar", "Default sidebar selected");
+  await SidebarUI.show("viewHistorySidebar");
+
+  await hideSidebar();
+  await showSidebar();
+  is(SidebarUI.currentID, "viewHistorySidebar", "Selected sidebar remembered");
+
+  await hideSidebar();
+  let otherWin = await BrowserTestUtils.openNewBrowserWindow();
+  await showSidebar(otherWin);
+  is(
+    otherWin.SidebarUI.currentID,
+    "viewHistorySidebar",
+    "Selected sidebar remembered across windows"
+  );
+  await hideSidebar(otherWin);
+
+  await BrowserTestUtils.closeWindow(otherWin);
+});
diff --git a/browser/components/customizableui/test/browser_switch_to_customize_mode.js b/browser/components/customizableui/test/browser_switch_to_customize_mode.js
new file mode 100644
index 0000000000..55e80d3517
--- /dev/null
+++ b/browser/components/customizableui/test/browser_switch_to_customize_mode.js
@@ -0,0 +1,53 @@
+"use strict";
+
+add_task(async function () {
+  await startCustomizing();
+  is(gBrowser.tabs.length, 2, "Should have 2 tabs");
+
+  let paletteKidCount = document.getElementById(
+    "customization-palette"
+  ).childElementCount;
+  let nonCustomizingTab = gBrowser.tabContainer.querySelector(
+    "tab:not([customizemode=true])"
+  );
+  let finishedCustomizing = BrowserTestUtils.waitForEvent(
+    gNavToolbox,
+    "aftercustomization"
+  );
+  await BrowserTestUtils.switchTab(gBrowser, nonCustomizingTab);
+  await finishedCustomizing;
+
+  let startedCount = 0;
+  let handler = e => startedCount++;
+  gNavToolbox.addEventListener("customizationstarting", handler);
+  await startCustomizing();
+  CustomizableUI.removeWidgetFromArea("stop-reload-button");
+  await gCustomizeMode.reset().catch(e => {
+    ok(
+      false,
+      "Threw an exception trying to reset after making modifications in customize mode: " +
+        e
+    );
+  });
+
+  let newKidCount = document.getElementById(
+    "customization-palette"
+  ).childElementCount;
+  is(
+    newKidCount,
+    paletteKidCount,
+    "Should have just as many items in the palette as before."
+  );
+  await endCustomizing();
+  is(startedCount, 1, "Should have only started once");
+  gNavToolbox.removeEventListener("customizationstarting", handler);
+  let customizableToolbars = document.querySelectorAll(
+    "toolbar[customizable=true]:not([autohide=true])"
+  );
+  for (let toolbar of customizableToolbars) {
+    ok(
+      !toolbar.hasAttribute("customizing"),
+      "Toolbar " + toolbar.id + " is no longer customizing"
+    );
+  }
+});
diff --git a/browser/components/customizableui/test/browser_synced_tabs_menu.js b/browser/components/customizableui/test/browser_synced_tabs_menu.js
new file mode 100644
index 0000000000..ff60167fea
--- /dev/null
+++ b/browser/components/customizableui/test/browser_synced_tabs_menu.js
@@ -0,0 +1,523 @@
+/* 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";
+
+requestLongerTimeout(2);
+
+const { FxAccounts } = ChromeUtils.importESModule(
+  "resource://gre/modules/FxAccounts.sys.mjs"
+);
+let { SyncedTabs } = ChromeUtils.importESModule(
+  "resource://services-sync/SyncedTabs.sys.mjs"
+);
+let { UIState } = ChromeUtils.importESModule(
+  "resource://services-sync/UIState.sys.mjs"
+);
+
+ChromeUtils.defineESModuleGetters(this, {
+  UITour: "resource:///modules/UITour.sys.mjs",
+});
+
+const DECKINDEX_TABS = 0;
+const DECKINDEX_FETCHING = 1;
+const DECKINDEX_TABSDISABLED = 2;
+const DECKINDEX_NOCLIENTS = 3;
+
+const SAMPLE_TAB_URL = "https://example.com/";
+
+var initialLocation = gBrowser.currentURI.spec;
+var newTab = null;
+
+// A helper to notify there are new tabs. Returns a promise that is resolved
+// once the UI has been updated.
+function updateTabsPanel() {
+  let promiseTabsUpdated = promiseObserverNotified(
+    "synced-tabs-menu:test:tabs-updated"
+  );
+  Services.obs.notifyObservers(null, SyncedTabs.TOPIC_TABS_CHANGED);
+  return promiseTabsUpdated;
+}
+
+// This is the mock we use for SyncedTabs.jsm - tests may override various
+// functions.
+let mockedInternal = {
+  get isConfiguredToSyncTabs() {
+    return true;
+  },
+  getTabClients() {
+    return Promise.resolve([]);
+  },
+  syncTabs() {
+    return Promise.resolve();
+  },
+  hasSyncedThisSession: false,
+};
+
+add_setup(async function () {
+  const getSignedInUser = FxAccounts.config.getSignedInUser;
+  FxAccounts.config.getSignedInUser = async () =>
+    Promise.resolve({ uid: "uid", email: "foo@bar.com" });
+  Services.prefs.setCharPref(
+    "identity.fxaccounts.remote.root",
+    "https://example.com/"
+  );
+
+  let oldInternal = SyncedTabs._internal;
+  SyncedTabs._internal = mockedInternal;
+
+  let origNotifyStateUpdated = UIState._internal.notifyStateUpdated;
+  // Sync start-up will interfere with our tests, don't let UIState send UI updates.
+  UIState._internal.notifyStateUpdated = () => {};
+
+  // Force gSync initialization
+  gSync.init();
+
+  registerCleanupFunction(() => {
+    FxAccounts.config.getSignedInUser = getSignedInUser;
+    Services.prefs.clearUserPref("identity.fxaccounts.remote.root");
+    UIState._internal.notifyStateUpdated = origNotifyStateUpdated;
+    SyncedTabs._internal = oldInternal;
+  });
+});
+
+// The test expects the about:preferences#sync page to open in the current tab
+async function openPrefsFromMenuPanel(expectedPanelId, entryPoint) {
+  info("Check Sync button functionality");
+  CustomizableUI.addWidgetToArea(
+    "sync-button",
+    CustomizableUI.AREA_FIXED_OVERFLOW_PANEL
+  );
+
+  await waitForOverflowButtonShown();
+
+  // check the button's functionality
+  await document.getElementById("nav-bar").overflowable.show();
+
+  if (entryPoint == "uitour") {
+    UITour.tourBrowsersByWindow.set(window, new Set());
+    UITour.tourBrowsersByWindow.get(window).add(gBrowser.selectedBrowser);
+  }
+
+  let syncButton = document.getElementById("sync-button");
+  ok(syncButton, "The Sync button was added to the Panel Menu");
+
+  let tabsUpdatedPromise = promiseObserverNotified(
+    "synced-tabs-menu:test:tabs-updated"
+  );
+  syncButton.click();
+  let syncPanel = document.getElementById("PanelUI-remotetabs");
+  let viewShownPromise = BrowserTestUtils.waitForEvent(syncPanel, "ViewShown");
+  await Promise.all([tabsUpdatedPromise, viewShownPromise]);
+  ok(syncPanel.getAttribute("visible"), "Sync Panel is in view");
+
+  // Sync is not configured - verify that state is reflected.
+  let subpanel = document.getElementById(expectedPanelId);
+  ok(!subpanel.hidden, "sync setup element is visible");
+
+  // Find and click the "setup" button.
+  let setupButton = subpanel.querySelector(".PanelUI-remotetabs-button");
+  setupButton.click();
+
+  await new Promise(resolve => {
+    let handler = async e => {
+      if (
+        e.originalTarget != gBrowser.selectedBrowser.contentDocument ||
+        e.target.location.href == "about:blank"
+      ) {
+        info("Skipping spurious 'load' event for " + e.target.location.href);
+        return;
+      }
+      gBrowser.selectedBrowser.removeEventListener("load", handler, true);
+      resolve();
+    };
+    gBrowser.selectedBrowser.addEventListener("load", handler, true);
+  });
+  newTab = gBrowser.selectedTab;
+
+  is(
+    gBrowser.currentURI.spec,
+    "about:preferences?entrypoint=" + entryPoint + "#sync",
+    "Firefox Sync preference page opened with `menupanel` entrypoint"
+  );
+  ok(!isOverflowOpen(), "The panel closed");
+
+  if (isOverflowOpen()) {
+    await hideOverflow();
+  }
+}
+
+function hideOverflow() {
+  let panelHidePromise = promiseOverflowHidden(window);
+  PanelUI.overflowPanel.hidePopup();
+  return panelHidePromise;
+}
+
+async function asyncCleanup() {
+  // reset the panel UI to the default state
+  await resetCustomization();
+  ok(CustomizableUI.inDefaultState, "The panel UI is in default state again.");
+
+  // restore the tabs
+  BrowserTestUtils.addTab(gBrowser, initialLocation);
+  gBrowser.removeTab(newTab);
+  UITour.tourBrowsersByWindow.delete(window);
+}
+
+// When Sync is not setup.
+add_task(async function () {
+  gSync.updateAllUI({ status: UIState.STATUS_NOT_CONFIGURED });
+  await openPrefsFromMenuPanel("PanelUI-remotetabs-setupsync", "synced-tabs");
+});
+add_task(asyncCleanup);
+
+// When an account is connected by Sync is not enabled.
+add_task(async function () {
+  gSync.updateAllUI({ status: UIState.STATUS_SIGNED_IN, syncEnabled: false });
+  await openPrefsFromMenuPanel(
+    "PanelUI-remotetabs-syncdisabled",
+    "synced-tabs"
+  );
+});
+add_task(asyncCleanup);
+
+// When Sync is configured in an unverified state.
+add_task(async function () {
+  gSync.updateAllUI({
+    status: UIState.STATUS_NOT_VERIFIED,
+    email: "foo@bar.com",
+  });
+  await openPrefsFromMenuPanel("PanelUI-remotetabs-unverified", "synced-tabs");
+});
+add_task(asyncCleanup);
+
+// When Sync is configured in a "needs reauthentication" state.
+add_task(async function () {
+  gSync.updateAllUI({
+    status: UIState.STATUS_LOGIN_FAILED,
+    email: "foo@bar.com",
+  });
+  await openPrefsFromMenuPanel("PanelUI-remotetabs-reauthsync", "synced-tabs");
+});
+
+// Test the Connect Another Device button
+add_task(async function () {
+  gSync.updateAllUI({
+    status: UIState.STATUS_SIGNED_IN,
+    syncEnabled: true,
+    email: "foo@bar.com",
+    lastSync: new Date(),
+  });
+
+  let button = document.getElementById(
+    "PanelUI-remotetabs-connect-device-button"
+  );
+  ok(button, "found the button");
+
+  await document.getElementById("nav-bar").overflowable.show();
+  let expectedUrl =
+    "https://example.com/connect_another_device?context=" +
+    "fx_desktop_v3&entrypoint=synced-tabs&service=sync&uid=uid&email=foo%40bar.com";
+  let promiseTabOpened = BrowserTestUtils.waitForNewTab(gBrowser, expectedUrl);
+  button.click();
+  // the panel should have been closed.
+  ok(!isOverflowOpen(), "click closed the panel");
+  await promiseTabOpened;
+
+  gBrowser.removeTab(gBrowser.selectedTab);
+});
+
+// Test the "Sync Now" button
+add_task(async function () {
+  gSync.updateAllUI({
+    status: UIState.STATUS_SIGNED_IN,
+    syncEnabled: true,
+    email: "foo@bar.com",
+    lastSync: new Date(),
+  });
+
+  await document.getElementById("nav-bar").overflowable.show();
+  let tabsUpdatedPromise = promiseObserverNotified(
+    "synced-tabs-menu:test:tabs-updated"
+  );
+  let syncPanel = document.getElementById("PanelUI-remotetabs");
+  let viewShownPromise = BrowserTestUtils.waitForEvent(syncPanel, "ViewShown");
+  let syncButton = document.getElementById("sync-button");
+  syncButton.click();
+  await Promise.all([tabsUpdatedPromise, viewShownPromise]);
+  ok(syncPanel.getAttribute("visible"), "Sync Panel is in view");
+
+  let subpanel = document.getElementById("PanelUI-remotetabs-main");
+  ok(!subpanel.hidden, "main pane is visible");
+  let deck = document.getElementById("PanelUI-remotetabs-deck");
+
+  // The widget is still fetching tabs, as we've neutered everything that
+  // provides them
+  is(deck.selectedIndex, DECKINDEX_FETCHING, "first deck entry is visible");
+
+  // Tell the widget there are tabs available, but with zero clients.
+  mockedInternal.getTabClients = () => {
+    return Promise.resolve([]);
+  };
+  mockedInternal.hasSyncedThisSession = true;
+  await updateTabsPanel();
+  // The UI should be showing the "no clients" pane.
+  is(
+    deck.selectedIndex,
+    DECKINDEX_NOCLIENTS,
+    "no-clients deck entry is visible"
+  );
+
+  // Tell the widget there are tabs available - we have 3 clients, one with no
+  // tabs.
+  mockedInternal.getTabClients = () => {
+    return Promise.resolve([
+      {
+        id: "guid_mobile",
+        type: "client",
+        name: "My Phone",
+        lastModified: 1492201200,
+        tabs: [],
+      },
+      {
+        id: "guid_desktop",
+        type: "client",
+        name: "My Desktop",
+        lastModified: 1492201200,
+        tabs: [
+          {
+            title: "http://example.com/10",
+            lastUsed: 10, // the most recent
+          },
+          {
+            title: "http://example.com/1",
+            lastUsed: 1, // the least recent.
+          },
+          {
+            title: "http://example.com/5",
+            lastUsed: 5,
+          },
+        ],
+      },
+      {
+        id: "guid_second_desktop",
+        name: "My Other Desktop",
+        lastModified: 1492201200,
+        tabs: [
+          {
+            title: "http://example.com/6",
+            lastUsed: 6,
+          },
+        ],
+      },
+    ]);
+  };
+  await updateTabsPanel();
+
+  // The UI should be showing tabs!
+  is(deck.selectedIndex, DECKINDEX_TABS, "no-clients deck entry is visible");
+  let tabList = document.getElementById("PanelUI-remotetabs-tabslist");
+  let node = tabList.firstElementChild;
+  // First entry should be the client with the most-recent tab.
+  is(node.nodeName, "vbox");
+  let currentClient = node;
+  node = node.firstElementChild;
+  is(node.getAttribute("itemtype"), "client", "node is a client entry");
+  is(node.textContent, "My Desktop", "correct client");
+  // Next entry is the most-recent tab
+  node = node.nextElementSibling;
+  is(node.getAttribute("itemtype"), "tab", "node is a tab");
+  is(node.getAttribute("label"), "http://example.com/10");
+
+  // Next entry is the next-most-recent tab
+  node = node.nextElementSibling;
+  is(node.getAttribute("itemtype"), "tab", "node is a tab");
+  is(node.getAttribute("label"), "http://example.com/5");
+
+  // Next entry is the least-recent tab from the first client.
+  node = node.nextElementSibling;
+  is(node.getAttribute("itemtype"), "tab", "node is a tab");
+  is(node.getAttribute("label"), "http://example.com/1");
+  node = node.nextElementSibling;
+  is(node, null, "no more siblings");
+
+  // Next is a toolbarseparator between the clients.
+  node = currentClient.nextElementSibling;
+  is(node.nodeName, "toolbarseparator");
+
+  // Next is the container for client 2.
+  node = node.nextElementSibling;
+  is(node.nodeName, "vbox");
+  currentClient = node;
+
+  // Next is the client with 1 tab.
+  node = node.firstElementChild;
+  is(node.getAttribute("itemtype"), "client", "node is a client entry");
+  is(node.textContent, "My Other Desktop", "correct client");
+  // Its single tab
+  node = node.nextElementSibling;
+  is(node.getAttribute("itemtype"), "tab", "node is a tab");
+  is(node.getAttribute("label"), "http://example.com/6");
+  node = node.nextElementSibling;
+  is(node, null, "no more siblings");
+
+  // Next is a toolbarseparator between the clients.
+  node = currentClient.nextElementSibling;
+  is(node.nodeName, "toolbarseparator");
+
+  // Next is the container for client 3.
+  node = node.nextElementSibling;
+  is(node.nodeName, "vbox");
+  currentClient = node;
+
+  // Next is the client with no tab.
+  node = node.firstElementChild;
+  is(node.getAttribute("itemtype"), "client", "node is a client entry");
+  is(node.textContent, "My Phone", "correct client");
+  // There is a single node saying there's no tabs for the client.
+  node = node.nextElementSibling;
+  is(node.nodeName, "label", "node is a label");
+  is(node.getAttribute("itemtype"), "", "node is neither a tab nor a client");
+
+  node = node.nextElementSibling;
+  is(node, null, "no more siblings");
+  is(currentClient.nextElementSibling, null, "no more clients");
+
+  // Check accessibility. There should be containers for each client, with an
+  // aria attribute that identifies the client name.
+  let clientContainers = [
+    ...tabList.querySelectorAll("[aria-labelledby]").values(),
+  ];
+  let labelIds = clientContainers.map(container =>
+    container.getAttribute("aria-labelledby")
+  );
+  let labels = labelIds.map(id => document.getElementById(id).textContent);
+  Assert.deepEqual(labels.sort(), [
+    "My Desktop",
+    "My Other Desktop",
+    "My Phone",
+  ]);
+
+  let didSync = false;
+  let oldDoSync = gSync.doSync;
+  gSync.doSync = function () {
+    didSync = true;
+    gSync.doSync = oldDoSync;
+  };
+
+  let syncNowButton = document.getElementById("PanelUI-remotetabs-syncnow");
+  is(syncNowButton.disabled, false);
+  syncNowButton.click();
+  ok(didSync, "clicking the button called the correct function");
+
+  await hideOverflow();
+});
+
+// Test the pagination capabilities (Show More/All tabs)
+add_task(async function () {
+  mockedInternal.getTabClients = () => {
+    return Promise.resolve([
+      {
+        id: "guid_desktop",
+        type: "client",
+        name: "My Desktop",
+        lastModified: 1492201200,
+        tabs: (function () {
+          let allTabsDesktop = [];
+          // We choose 77 tabs, because TABS_PER_PAGE is 25, which means
+          // on the second to last page we should have 22 items shown
+          // (because we have to show at least NEXT_PAGE_MIN_TABS=5 tabs on the last page)
+          for (let i = 1; i <= 77; i++) {
+            allTabsDesktop.push({ title: "Tab #" + i, url: SAMPLE_TAB_URL });
+          }
+          return allTabsDesktop;
+        })(),
+      },
+    ]);
+  };
+
+  gSync.updateAllUI({
+    status: UIState.STATUS_SIGNED_IN,
+    syncEnabled: true,
+    lastSync: new Date(),
+    email: "foo@bar.com",
+  });
+
+  await document.getElementById("nav-bar").overflowable.show();
+  let tabsUpdatedPromise = promiseObserverNotified(
+    "synced-tabs-menu:test:tabs-updated"
+  );
+  let syncPanel = document.getElementById("PanelUI-remotetabs");
+  let viewShownPromise = BrowserTestUtils.waitForEvent(syncPanel, "ViewShown");
+  let syncButton = document.getElementById("sync-button");
+  syncButton.click();
+  await Promise.all([tabsUpdatedPromise, viewShownPromise]);
+
+  // Check pre-conditions
+  ok(syncPanel.getAttribute("visible"), "Sync Panel is in view");
+  let subpanel = document.getElementById("PanelUI-remotetabs-main");
+  ok(!subpanel.hidden, "main pane is visible");
+  let deck = document.getElementById("PanelUI-remotetabs-deck");
+  is(deck.selectedIndex, DECKINDEX_TABS, "we should be showing tabs");
+
+  function checkTabsPage(tabsShownCount, showMoreLabel) {
+    let tabList = document.getElementById("PanelUI-remotetabs-tabslist");
+    let node = tabList.firstElementChild.firstElementChild;
+    is(node.getAttribute("itemtype"), "client", "node is a client entry");
+    is(node.textContent, "My Desktop", "correct client");
+    for (let i = 0; i < tabsShownCount; i++) {
+      node = node.nextElementSibling;
+      is(node.getAttribute("itemtype"), "tab", "node is a tab");
+      is(
+        node.getAttribute("label"),
+        "Tab #" + (i + 1),
+        "the tab is the correct one"
+      );
+      is(
+        node.getAttribute("targetURI"),
+        SAMPLE_TAB_URL,
+        "url is the correct one"
+      );
+    }
+    let showMoreButton;
+    if (showMoreLabel) {
+      node = showMoreButton = node.nextElementSibling;
+      is(
+        node.getAttribute("itemtype"),
+        "showmorebutton",
+        "node is a show more button"
+      );
+      is(node.getAttribute("label"), showMoreLabel);
+    }
+    node = node.nextElementSibling;
+    is(node, null, "no more entries");
+
+    return showMoreButton;
+  }
+
+  async function checkCanOpenURL() {
+    let tabList = document.getElementById("PanelUI-remotetabs-tabslist");
+    let node = tabList.firstElementChild.firstElementChild.nextElementSibling;
+    let promiseTabOpened = BrowserTestUtils.waitForLocationChange(
+      gBrowser,
+      SAMPLE_TAB_URL
+    );
+    node.click();
+    await promiseTabOpened;
+  }
+
+  let showMoreButton;
+  function clickShowMoreButton() {
+    let promise = promiseObserverNotified("synced-tabs-menu:test:tabs-updated");
+    showMoreButton.click();
+    return promise;
+  }
+
+  showMoreButton = checkTabsPage(25, "Show More Tabs");
+  await clickShowMoreButton();
+
+  checkTabsPage(77, null);
+  /* calling this will close the overflow menu */
+  await checkCanOpenURL();
+});
diff --git a/browser/components/customizableui/test/browser_tabbar_big_widgets.js b/browser/components/customizableui/test/browser_tabbar_big_widgets.js
new file mode 100644
index 0000000000..3722ea9cdc
--- /dev/null
+++ b/browser/components/customizableui/test/browser_tabbar_big_widgets.js
@@ -0,0 +1,32 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/
+ */
+
+const kButtonId = "test-tabbar-size-with-large-buttons";
+
+function test() {
+  registerCleanupFunction(cleanup);
+  let titlebar = document.getElementById("titlebar");
+  let originalHeight = titlebar.getBoundingClientRect().height;
+  let button = document.createXULElement("toolbarbutton");
+  button.id = kButtonId;
+  button.setAttribute("style", "min-height: 100px");
+  gNavToolbox.palette.appendChild(button);
+  CustomizableUI.addWidgetToArea(kButtonId, CustomizableUI.AREA_TABSTRIP);
+  let currentHeight = titlebar.getBoundingClientRect().height;
+  ok(currentHeight > originalHeight, "Titlebar should have grown");
+  CustomizableUI.removeWidgetFromArea(kButtonId);
+  currentHeight = titlebar.getBoundingClientRect().height;
+  is(
+    currentHeight,
+    originalHeight,
+    "Titlebar should have gone back to its original size."
+  );
+}
+
+function cleanup() {
+  let btn = document.getElementById(kButtonId);
+  if (btn) {
+    btn.remove();
+  }
+}
diff --git a/browser/components/customizableui/test/browser_toolbar_collapsed_states.js b/browser/components/customizableui/test/browser_toolbar_collapsed_states.js
new file mode 100644
index 0000000000..23da60a7d5
--- /dev/null
+++ b/browser/components/customizableui/test/browser_toolbar_collapsed_states.js
@@ -0,0 +1,112 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Checks that CustomizableUI reports the expected collapsed toolbar IDs.
+ *
+ * Note: on macOS, expectations for CustomizableUI.AREA_MENUBAR are
+ * automatically skipped since that area isn't available on that platform.
+ *
+ * @param {string[]} The IDs of the expected collapsed toolbars.
+ */
+function assertCollapsedToolbarIds(expected) {
+  if (AppConstants.platform == "macosx") {
+    let menubarIndex = expected.indexOf(CustomizableUI.AREA_MENUBAR);
+    if (menubarIndex != -1) {
+      expected.splice(menubarIndex, 1);
+    }
+  }
+
+  let collapsedIds = CustomizableUI.getCollapsedToolbarIds(window);
+  Assert.equal(collapsedIds.size, expected.length);
+  for (let expectedId of expected) {
+    Assert.ok(
+      collapsedIds.has(expectedId),
+      `${expectedId} should be collapsed`
+    );
+  }
+}
+
+registerCleanupFunction(async () => {
+  await CustomizableUI.reset();
+});
+
+/**
+ * Tests that CustomizableUI.getCollapsedToolbarIds will return the IDs of
+ * toolbars that are collapsed, or menubars that are autohidden.
+ */
+add_task(async function test_toolbar_collapsed_states() {
+  // By default, we expect the menubar and the bookmarks toolbar to be
+  // collapsed.
+  assertCollapsedToolbarIds([
+    CustomizableUI.AREA_BOOKMARKS,
+    CustomizableUI.AREA_MENUBAR,
+  ]);
+
+  let bookmarksToolbar = document.getElementById(CustomizableUI.AREA_BOOKMARKS);
+  // Make sure we're configured to show the bookmarks toolbar on about:newtab.
+  setToolbarVisibility(bookmarksToolbar, "newtab");
+
+  let newTab = await BrowserTestUtils.openNewForegroundTab({
+    gBrowser,
+    opening: "about:newtab",
+    waitForLoad: false,
+  });
+  // Now that we've opened about:newtab, the bookmarks toolbar should now
+  // be visible.
+  assertCollapsedToolbarIds([CustomizableUI.AREA_MENUBAR]);
+  await BrowserTestUtils.removeTab(newTab);
+
+  // And with about:newtab closed again, the bookmarks toolbar should be
+  // reported as collapsed.
+  assertCollapsedToolbarIds([
+    CustomizableUI.AREA_BOOKMARKS,
+    CustomizableUI.AREA_MENUBAR,
+  ]);
+
+  // Make sure we're configured to show the bookmarks toolbar on about:newtab.
+  setToolbarVisibility(bookmarksToolbar, "always");
+  assertCollapsedToolbarIds([CustomizableUI.AREA_MENUBAR]);
+
+  setToolbarVisibility(bookmarksToolbar, "never");
+  assertCollapsedToolbarIds([
+    CustomizableUI.AREA_BOOKMARKS,
+    CustomizableUI.AREA_MENUBAR,
+  ]);
+
+  if (AppConstants.platform != "macosx") {
+    // We'll still consider the menubar collapsed by default, even if it's being temporarily
+    // shown via the alt key.
+    let menubarActive = BrowserTestUtils.waitForEvent(
+      window,
+      "DOMMenuBarActive"
+    );
+    EventUtils.synthesizeKey("VK_ALT", {});
+    await menubarActive;
+    assertCollapsedToolbarIds([
+      CustomizableUI.AREA_BOOKMARKS,
+      CustomizableUI.AREA_MENUBAR,
+    ]);
+    let menubarInactive = BrowserTestUtils.waitForEvent(
+      window,
+      "DOMMenuBarInactive"
+    );
+    EventUtils.synthesizeKey("VK_ESCAPE", {});
+    await menubarInactive;
+    assertCollapsedToolbarIds([
+      CustomizableUI.AREA_BOOKMARKS,
+      CustomizableUI.AREA_MENUBAR,
+    ]);
+
+    let menubar = document.getElementById(CustomizableUI.AREA_MENUBAR);
+    setToolbarVisibility(menubar, true);
+    assertCollapsedToolbarIds([CustomizableUI.AREA_BOOKMARKS]);
+    setToolbarVisibility(menubar, false);
+    assertCollapsedToolbarIds([
+      CustomizableUI.AREA_BOOKMARKS,
+      CustomizableUI.AREA_MENUBAR,
+    ]);
+  }
+});
diff --git a/browser/components/customizableui/test/browser_touchbar_customization.js b/browser/components/customizableui/test/browser_touchbar_customization.js
new file mode 100644
index 0000000000..106c202ad9
--- /dev/null
+++ b/browser/components/customizableui/test/browser_touchbar_customization.js
@@ -0,0 +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";
+
+// Checks if the Customize Touch Bar button appears when a Touch Bar is
+// initialized.
+add_task(async function customizeTouchBarButtonAppears() {
+  let updater = Cc["@mozilla.org/widget/touchbarupdater;1"].getService(
+    Ci.nsITouchBarUpdater
+  );
+  // This value will be reset to its default the next time a window is opened.
+  updater.setTouchBarInitialized(true);
+  await startCustomizing();
+  let touchbarButton = document.querySelector("#customization-touchbar-button");
+  ok(!touchbarButton.hidden, "Customize Touch Bar button is not hidden.");
+  let touchbarSpacer = document.querySelector("#customization-touchbar-spacer");
+  ok(!touchbarSpacer.hidden, "Customize Touch Bar spacer is not hidden.");
+  await endCustomizing();
+});
diff --git a/browser/components/customizableui/test/browser_unified_extensions_reset.js b/browser/components/customizableui/test/browser_unified_extensions_reset.js
new file mode 100644
index 0000000000..fdee8cf76a
--- /dev/null
+++ b/browser/components/customizableui/test/browser_unified_extensions_reset.js
@@ -0,0 +1,91 @@
+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Tests that if Unified Extensions UI is enabled that resetting the toolbars
+ * puts all browser action buttons into the AREA_ADDONS area.
+ */
+add_task(async function test_reset_with_unified_extensions_ui() {
+  const kWebExtensionWidgetIDs = [
+    "ext0-browser-action",
+    "ext1-browser-action",
+    "ext2-browser-action",
+    "ext3-browser-action",
+    "ext4-browser-action",
+    "ext5-browser-action",
+    "ext6-browser-action",
+    "ext7-browser-action",
+    "ext8-browser-action",
+    "ext9-browser-action",
+    "ext10-browser-action",
+  ];
+
+  for (let widgetID of kWebExtensionWidgetIDs) {
+    CustomizableUI.createWidget({
+      id: widgetID,
+      label: "Test extension widget",
+      defaultArea: CustomizableUI.AREA_NAVBAR,
+      webExtension: true,
+    });
+  }
+
+  // Now let's put these browser actions in a bunch of different places.
+  // Regardless of where they go, we're going to expect them in AREA_ADDONS
+  // after we reset.
+  CustomizableUI.addWidgetToArea(
+    kWebExtensionWidgetIDs[0],
+    CustomizableUI.AREA_TABSTRIP
+  );
+  CustomizableUI.addWidgetToArea(
+    kWebExtensionWidgetIDs[1],
+    CustomizableUI.AREA_TABSTRIP
+  );
+
+  // macOS doesn't have AREA_MENUBAR registered, so we'll leave these widgets
+  // behind in the AREA_NAVBAR there, and put them into the menubar on the
+  // other platforms.
+  if (AppConstants.platform != "macosx") {
+    CustomizableUI.addWidgetToArea(
+      kWebExtensionWidgetIDs[2],
+      CustomizableUI.AREA_MENUBAR
+    );
+    CustomizableUI.addWidgetToArea(
+      kWebExtensionWidgetIDs[3],
+      CustomizableUI.AREA_MENUBAR
+    );
+  }
+
+  CustomizableUI.addWidgetToArea(
+    kWebExtensionWidgetIDs[4],
+    CustomizableUI.AREA_BOOKMARKS
+  );
+  CustomizableUI.addWidgetToArea(
+    kWebExtensionWidgetIDs[5],
+    CustomizableUI.AREA_BOOKMARKS
+  );
+  CustomizableUI.addWidgetToArea(
+    kWebExtensionWidgetIDs[6],
+    CustomizableUI.AREA_ADDONS
+  );
+  CustomizableUI.addWidgetToArea(
+    kWebExtensionWidgetIDs[7],
+    CustomizableUI.AREA_ADDONS
+  );
+
+  CustomizableUI.reset();
+
+  // Let's force the Unified Extensions panel to register itself now if it
+  // wasn't already done. Using the getter should be sufficient.
+  Assert.ok(gUnifiedExtensions.panel, "Should have found the panel.");
+
+  for (let widgetID of kWebExtensionWidgetIDs) {
+    let { area } = CustomizableUI.getPlacementOfWidget(widgetID);
+    Assert.equal(area, CustomizableUI.AREA_ADDONS);
+    // Let's double-check that they're actually in there in the DOM too.
+    let widget = CustomizableUI.getWidget(widgetID).forWindow(window);
+    Assert.equal(widget.node.parentElement.id, CustomizableUI.AREA_ADDONS);
+    CustomizableUI.destroyWidget(widgetID);
+  }
+});
diff --git a/browser/components/customizableui/test/browser_widget_animation.js b/browser/components/customizableui/test/browser_widget_animation.js
new file mode 100644
index 0000000000..514e3f763b
--- /dev/null
+++ b/browser/components/customizableui/test/browser_widget_animation.js
@@ -0,0 +1,84 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+gReduceMotionOverride = false;
+
+function promiseWidgetAnimationOut(aNode) {
+  let animationNode = aNode;
+  if (
+    animationNode.tagName != "toolbaritem" &&
+    animationNode.tagName != "toolbarbutton"
+  ) {
+    animationNode = animationNode.closest("toolbaritem");
+  }
+  if (animationNode.parentNode.id.startsWith("wrapper-")) {
+    animationNode = animationNode.parentNode;
+  }
+  return new Promise(resolve => {
+    animationNode.addEventListener(
+      "animationend",
+      function cleanupWidgetAnimationOut(e) {
+        if (
+          e.animationName == "widget-animate-out" &&
+          e.target.id == animationNode.id
+        ) {
+          animationNode.removeEventListener(
+            "animationend",
+            cleanupWidgetAnimationOut
+          );
+          ok(true, "The widget`s animationend should have happened");
+          resolve();
+        }
+      }
+    );
+  });
+}
+
+function promiseOverflowAnimationEnd() {
+  return new Promise(resolve => {
+    let overflowButton = document.getElementById("nav-bar-overflow-button");
+    overflowButton.addEventListener(
+      "animationend",
+      function cleanupOverflowAnimationOut(event) {
+        if (event.animationName == "overflow-animation") {
+          overflowButton.removeEventListener(
+            "animationend",
+            cleanupOverflowAnimationOut
+          );
+          ok(
+            true,
+            "The overflow button`s animationend event should have happened"
+          );
+          resolve();
+        }
+      }
+    );
+  });
+}
+
+// Right-click on the stop/reload button, use the context menu to move it to the overflow menu.
+// The button should animate out, and the overflow menu should animate upon adding.
+add_task(async function () {
+  let stopReloadButton = document.getElementById("stop-reload-button");
+  let contextMenu = document.getElementById("toolbar-context-menu");
+  let shownPromise = popupShown(contextMenu);
+  EventUtils.synthesizeMouseAtCenter(stopReloadButton, {
+    type: "contextmenu",
+    button: 2,
+  });
+  await shownPromise;
+
+  contextMenu.activateItem(
+    contextMenu.querySelector(".customize-context-moveToPanel")
+  );
+
+  await Promise.all([
+    promiseWidgetAnimationOut(stopReloadButton),
+    promiseOverflowAnimationEnd(),
+  ]);
+  ok(true, "The widget and overflow animations should have both happened.");
+});
+
+registerCleanupFunction(CustomizableUI.reset);
diff --git a/browser/components/customizableui/test/browser_widget_recreate_events.js b/browser/components/customizableui/test/browser_widget_recreate_events.js
new file mode 100644
index 0000000000..3eca9231a8
--- /dev/null
+++ b/browser/components/customizableui/test/browser_widget_recreate_events.js
@@ -0,0 +1,99 @@
+"use strict";
+
+const widgetData = {
+  id: "test-widget",
+  type: "view",
+  viewId: "PanelUI-testbutton",
+  label: "test widget label",
+  onViewShowing() {},
+  onViewHiding() {},
+};
+
+async function simulateWidgetOpen() {
+  let testWidgetButton = document.getElementById("test-widget");
+  let testWidgetShowing = BrowserTestUtils.waitForEvent(
+    document,
+    "popupshowing",
+    true
+  );
+  testWidgetButton.click();
+  await testWidgetShowing;
+}
+
+async function simulateWidgetClose() {
+  let panel = document.getElementById("customizationui-widget-panel");
+  let panelHidden = BrowserTestUtils.waitForEvent(panel, "popuphidden");
+
+  panel.hidePopup();
+  await panelHidden;
+}
+
+function createPanelView() {
+  let panelView = document.createXULElement("panelview");
+  panelView.id = "PanelUI-testbutton";
+  let vbox = document.createXULElement("vbox");
+  panelView.appendChild(vbox);
+  return panelView;
+}
+
+/**
+ * Check that panel view/hide events are added back,
+ * if widget is destroyed and created again in one session.
+ */
+add_task(async function () {
+  let viewCache = document.getElementById("appMenu-viewCache");
+  let panelView = createPanelView();
+  viewCache.appendChild(panelView);
+
+  CustomizableUI.createWidget(widgetData);
+  CustomizableUI.addWidgetToArea("test-widget", "nav-bar");
+
+  // Simulate clicking and wait for the open
+  // so we ensure the lazy event creation is done.
+  await simulateWidgetOpen();
+
+  let listeners = Services.els.getListenerInfoFor(panelView);
+  ok(
+    listeners.some(info => info.type == "ViewShowing"),
+    "ViewShowing event added"
+  );
+  ok(
+    listeners.some(info => info.type == "ViewHiding"),
+    "ViewHiding event added"
+  );
+
+  await simulateWidgetClose();
+  CustomizableUI.destroyWidget("test-widget");
+
+  listeners = Services.els.getListenerInfoFor(panelView);
+  // Ensure the events got removed after destorying the widget.
+  ok(
+    !listeners.some(info => info.type == "ViewShowing"),
+    "ViewShowing event removed"
+  );
+  ok(
+    !listeners.some(info => info.type == "ViewHiding"),
+    "ViewHiding event removed"
+  );
+
+  CustomizableUI.createWidget(widgetData);
+  // Simulate clicking and wait for the open
+  // so we ensure the lazy event creation is done.
+  // We need to do this again because we destroyed the widget.
+  await simulateWidgetOpen();
+
+  listeners = Services.els.getListenerInfoFor(panelView);
+  ok(
+    listeners.some(info => info.type == "ViewShowing"),
+    "ViewShowing event added again"
+  );
+  ok(
+    listeners.some(info => info.type == "ViewHiding"),
+    "ViewHiding event added again"
+  );
+
+  await simulateWidgetClose();
+  CustomizableUI.destroyWidget("test-widget");
+  panelView.remove();
+  CustomizableUI.reset();
+});
diff --git a/browser/components/customizableui/test/dummy_history_item.html b/browser/components/customizableui/test/dummy_history_item.html
new file mode 100644
index 0000000000..23a6992923
--- /dev/null
+++ b/browser/components/customizableui/test/dummy_history_item.html
@@ -0,0 +1,2 @@
+<title>Happy History Hero</title>
+<p>I am a page for the history books.</p>
diff --git a/browser/components/customizableui/test/head.js b/browser/components/customizableui/test/head.js
new file mode 100644
index 0000000000..cf1b86d929
--- /dev/null
+++ b/browser/components/customizableui/test/head.js
@@ -0,0 +1,536 @@
+/* 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";
+
+ChromeUtils.defineESModuleGetters(this, {
+  CustomizableUI: "resource:///modules/CustomizableUI.sys.mjs",
+  CustomizableUITestUtils:
+    "resource://testing-common/CustomizableUITestUtils.sys.mjs",
+});
+
+var EventUtils = {};
+Services.scriptloader.loadSubScript(
+  "chrome://mochikit/content/tests/SimpleTest/EventUtils.js",
+  EventUtils
+);
+
+/**
+ * Instance of CustomizableUITestUtils for the current browser window.
+ */
+var gCUITestUtils = new CustomizableUITestUtils(window);
+
+Services.prefs.setBoolPref("browser.uiCustomization.skipSourceNodeCheck", true);
+registerCleanupFunction(() =>
+  Services.prefs.clearUserPref("browser.uiCustomization.skipSourceNodeCheck")
+);
+
+var { synthesizeDrop, synthesizeMouseAtCenter } = EventUtils;
+
+const kForceOverflowWidthPx = 450;
+
+function createDummyXULButton(id, label, win = window) {
+  let btn = win.document.createXULElement("toolbarbutton");
+  btn.id = id;
+  btn.setAttribute("label", label || id);
+  btn.className = "toolbarbutton-1 chromeclass-toolbar-additional";
+  win.gNavToolbox.palette.appendChild(btn);
+  return btn;
+}
+
+var gAddedToolbars = new Set();
+
+function createToolbarWithPlacements(id, placements = [], properties = {}) {
+  gAddedToolbars.add(id);
+  let tb = document.createXULElement("toolbar");
+  tb.id = id;
+  tb.setAttribute("customizable", "true");
+
+  properties.type = CustomizableUI.TYPE_TOOLBAR;
+  properties.defaultPlacements = placements;
+  CustomizableUI.registerArea(id, properties);
+  gNavToolbox.appendChild(tb);
+  CustomizableUI.registerToolbarNode(tb);
+  return tb;
+}
+
+function createOverflowableToolbarWithPlacements(id, placements) {
+  gAddedToolbars.add(id);
+
+  let tb = document.createXULElement("toolbar");
+  tb.id = id;
+  tb.setAttribute("customizationtarget", id + "-target");
+
+  let customizationtarget = document.createXULElement("hbox");
+  customizationtarget.id = id + "-target";
+  customizationtarget.setAttribute("flex", "1");
+  tb.appendChild(customizationtarget);
+
+  let overflowPanel = document.createXULElement("panel");
+  overflowPanel.id = id + "-overflow";
+  document.getElementById("mainPopupSet").appendChild(overflowPanel);
+
+  let overflowList = document.createXULElement("vbox");
+  overflowList.id = id + "-overflow-list";
+  overflowPanel.appendChild(overflowList);
+
+  let chevron = document.createXULElement("toolbarbutton");
+  chevron.id = id + "-chevron";
+  tb.appendChild(chevron);
+
+  CustomizableUI.registerArea(id, {
+    type: CustomizableUI.TYPE_TOOLBAR,
+    defaultPlacements: placements,
+    overflowable: true,
+  });
+
+  tb.setAttribute("customizable", "true");
+  tb.setAttribute("overflowable", "true");
+  tb.setAttribute("default-overflowpanel", overflowPanel.id);
+  tb.setAttribute("default-overflowtarget", overflowList.id);
+  tb.setAttribute("default-overflowbutton", chevron.id);
+  tb.setAttribute("addon-webext-overflowbutton", "unified-extensions-button");
+  tb.setAttribute("addon-webext-overflowtarget", "overflowed-extensions-list");
+
+  gNavToolbox.appendChild(tb);
+  CustomizableUI.registerToolbarNode(tb);
+  return tb;
+}
+
+function removeCustomToolbars() {
+  CustomizableUI.reset();
+  for (let toolbarId of gAddedToolbars) {
+    CustomizableUI.unregisterArea(toolbarId, true);
+    let tb = document.getElementById(toolbarId);
+    if (tb.hasAttribute("overflowpanel")) {
+      let panel = document.getElementById(tb.getAttribute("overflowpanel"));
+      if (panel) {
+        panel.remove();
+      }
+    }
+    tb.remove();
+  }
+  gAddedToolbars.clear();
+}
+
+function resetCustomization() {
+  return CustomizableUI.reset();
+}
+
+function isInDevEdition() {
+  return AppConstants.MOZ_DEV_EDITION;
+}
+
+function removeNonReleaseButtons(areaPanelPlacements) {
+  if (isInDevEdition() && areaPanelPlacements.includes("developer-button")) {
+    areaPanelPlacements.splice(
+      areaPanelPlacements.indexOf("developer-button"),
+      1
+    );
+  }
+}
+
+function removeNonOriginalButtons() {
+  CustomizableUI.removeWidgetFromArea("sync-button");
+}
+
+function assertAreaPlacements(areaId, expectedPlacements) {
+  let actualPlacements = getAreaWidgetIds(areaId);
+  placementArraysEqual(areaId, actualPlacements, expectedPlacements);
+}
+
+function placementArraysEqual(areaId, actualPlacements, expectedPlacements) {
+  info("Actual placements: " + actualPlacements.join(", "));
+  info("Expected placements: " + expectedPlacements.join(", "));
+  is(
+    actualPlacements.length,
+    expectedPlacements.length,
+    "Area " + areaId + " should have " + expectedPlacements.length + " items."
+  );
+  let minItems = Math.min(expectedPlacements.length, actualPlacements.length);
+  for (let i = 0; i < minItems; i++) {
+    if (typeof expectedPlacements[i] == "string") {
+      is(
+        actualPlacements[i],
+        expectedPlacements[i],
+        "Item " + i + " in " + areaId + " should match expectations."
+      );
+    } else if (expectedPlacements[i] instanceof RegExp) {
+      ok(
+        expectedPlacements[i].test(actualPlacements[i]),
+        "Item " +
+          i +
+          " (" +
+          actualPlacements[i] +
+          ") in " +
+          areaId +
+          " should match " +
+          expectedPlacements[i]
+      );
+    } else {
+      ok(
+        false,
+        "Unknown type of expected placement passed to " +
+          " assertAreaPlacements. Is your test broken?"
+      );
+    }
+  }
+}
+
+function todoAssertAreaPlacements(areaId, expectedPlacements) {
+  let actualPlacements = getAreaWidgetIds(areaId);
+  let isPassing = actualPlacements.length == expectedPlacements.length;
+  let minItems = Math.min(expectedPlacements.length, actualPlacements.length);
+  for (let i = 0; i < minItems; i++) {
+    if (typeof expectedPlacements[i] == "string") {
+      isPassing = isPassing && actualPlacements[i] == expectedPlacements[i];
+    } else if (expectedPlacements[i] instanceof RegExp) {
+      isPassing = isPassing && expectedPlacements[i].test(actualPlacements[i]);
+    } else {
+      ok(
+        false,
+        "Unknown type of expected placement passed to " +
+          " assertAreaPlacements. Is your test broken?"
+      );
+    }
+  }
+  todo(
+    isPassing,
+    "The area placements for " +
+      areaId +
+      " should equal the expected placements."
+  );
+}
+
+function getAreaWidgetIds(areaId) {
+  return CustomizableUI.getWidgetIdsInArea(areaId);
+}
+
+function simulateItemDrag(aToDrag, aTarget, aEvent = {}, aOffset = 2) {
+  let ev = aEvent;
+  if (ev == "end" || ev == "start") {
+    let win = aTarget.ownerGlobal;
+    const dwu = win.windowUtils;
+    let bounds = dwu.getBoundsWithoutFlushing(aTarget);
+    if (ev == "end") {
+      ev = {
+        clientX: bounds.right - aOffset,
+        clientY: bounds.bottom - aOffset,
+      };
+    } else {
+      ev = { clientX: bounds.left + aOffset, clientY: bounds.top + aOffset };
+    }
+  }
+  ev._domDispatchOnly = true;
+  synthesizeDrop(
+    aToDrag.parentNode,
+    aTarget,
+    null,
+    null,
+    aToDrag.ownerGlobal,
+    aTarget.ownerGlobal,
+    ev
+  );
+  // Ensure dnd suppression is cleared.
+  synthesizeMouseAtCenter(aTarget, { type: "mouseup" }, aTarget.ownerGlobal);
+}
+
+function endCustomizing(aWindow = window) {
+  if (aWindow.document.documentElement.getAttribute("customizing") != "true") {
+    return true;
+  }
+  let afterCustomizationPromise = BrowserTestUtils.waitForEvent(
+    aWindow.gNavToolbox,
+    "aftercustomization"
+  );
+  aWindow.gCustomizeMode.exit();
+  return afterCustomizationPromise;
+}
+
+function startCustomizing(aWindow = window) {
+  if (aWindow.document.documentElement.getAttribute("customizing") == "true") {
+    return null;
+  }
+  let customizationReadyPromise = BrowserTestUtils.waitForEvent(
+    aWindow.gNavToolbox,
+    "customizationready"
+  );
+  aWindow.gCustomizeMode.enter();
+  return customizationReadyPromise;
+}
+
+function promiseObserverNotified(aTopic) {
+  return new Promise(resolve => {
+    Services.obs.addObserver(function onNotification(subject, topic, data) {
+      Services.obs.removeObserver(onNotification, topic);
+      resolve({ subject, data });
+    }, aTopic);
+  });
+}
+
+function openAndLoadWindow(aOptions, aWaitForDelayedStartup = false) {
+  return new Promise(resolve => {
+    let win = OpenBrowserWindow(aOptions);
+    if (aWaitForDelayedStartup) {
+      Services.obs.addObserver(function onDS(aSubject, aTopic, aData) {
+        if (aSubject != win) {
+          return;
+        }
+        Services.obs.removeObserver(onDS, "browser-delayed-startup-finished");
+        resolve(win);
+      }, "browser-delayed-startup-finished");
+    } else {
+      win.addEventListener(
+        "load",
+        function () {
+          resolve(win);
+        },
+        { once: true }
+      );
+    }
+  });
+}
+
+function promiseWindowClosed(win) {
+  return new Promise(resolve => {
+    win.addEventListener(
+      "unload",
+      function () {
+        resolve();
+      },
+      { once: true }
+    );
+    win.close();
+  });
+}
+
+function promiseOverflowShown(win) {
+  let panelEl = win.document.getElementById("widget-overflow");
+  return promisePanelElementShown(win, panelEl);
+}
+
+function promisePanelElementShown(win, aPanel) {
+  return new Promise((resolve, reject) => {
+    let timeoutId = win.setTimeout(() => {
+      reject("Panel did not show within 20 seconds.");
+    }, 20000);
+    function onPanelOpen(e) {
+      aPanel.removeEventListener("popupshown", onPanelOpen);
+      win.clearTimeout(timeoutId);
+      resolve();
+    }
+    aPanel.addEventListener("popupshown", onPanelOpen);
+  });
+}
+
+function promiseOverflowHidden(win) {
+  let panelEl = win.PanelUI.overflowPanel;
+  return promisePanelElementHidden(win, panelEl);
+}
+
+function promisePanelElementHidden(win, aPanel) {
+  return new Promise((resolve, reject) => {
+    let timeoutId = win.setTimeout(() => {
+      reject("Panel did not hide within 20 seconds.");
+    }, 20000);
+    function onPanelClose(e) {
+      aPanel.removeEventListener("popuphidden", onPanelClose);
+      win.clearTimeout(timeoutId);
+      executeSoon(resolve);
+    }
+    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 onViewShown(e) {
+      aSubview.removeEventListener("ViewShown", onViewShown);
+      win.clearTimeout(timeoutId);
+      resolve();
+    }
+    aSubview.addEventListener("ViewShown", onViewShown);
+  });
+}
+
+function subviewHidden(aSubview) {
+  return new Promise((resolve, reject) => {
+    let win = aSubview.ownerGlobal;
+    let timeoutId = win.setTimeout(() => {
+      reject("Subview (" + aSubview.id + ") did not hide within 20 seconds.");
+    }, 20000);
+    function onViewHiding(e) {
+      aSubview.removeEventListener("ViewHiding", onViewHiding);
+      win.clearTimeout(timeoutId);
+      resolve();
+    }
+    aSubview.addEventListener("ViewHiding", onViewHiding);
+  });
+}
+
+function waitFor(aTimeout = 100) {
+  return new Promise(resolve => {
+    setTimeout(() => resolve(), aTimeout);
+  });
+}
+
+/**
+ * Starts a load in an existing tab and waits for it to finish (via some event).
+ *
+ * @param aTab       The tab to load into.
+ * @param aUrl       The url to load.
+ * @param aEventType The load event type to wait for.  Defaults to "load".
+ * @return {Promise} resolved when the event is handled.
+ */
+function promiseTabLoadEvent(aTab, aURL) {
+  let browser = aTab.linkedBrowser;
+
+  BrowserTestUtils.loadURIString(browser, aURL);
+  return BrowserTestUtils.browserLoaded(browser);
+}
+
+/**
+ * Wait for an attribute on a node to change
+ *
+ * @param aNode      Node on which the mutation is expected
+ * @param aAttribute The attribute we're interested in
+ * @param aFilterFn  A function to check if the new value is what we want.
+ * @return {Promise} resolved when the requisite mutation shows up.
+ */
+function promiseAttributeMutation(aNode, aAttribute, aFilterFn) {
+  return new Promise((resolve, reject) => {
+    info("waiting for mutation of attribute '" + aAttribute + "'.");
+    let obs = new MutationObserver(mutations => {
+      for (let mut of mutations) {
+        let attr = mut.attributeName;
+        let newValue = mut.target.getAttribute(attr);
+        if (aFilterFn(newValue)) {
+          ok(
+            true,
+            "mutation occurred: attribute '" +
+              attr +
+              "' changed to '" +
+              newValue +
+              "' from '" +
+              mut.oldValue +
+              "'."
+          );
+          obs.disconnect();
+          resolve();
+        } else {
+          info(
+            "Ignoring mutation that produced value " +
+              newValue +
+              " because of filter."
+          );
+        }
+      }
+    });
+    obs.observe(aNode, { attributeFilter: [aAttribute] });
+  });
+}
+
+function popupShown(aPopup) {
+  return BrowserTestUtils.waitForPopupEvent(aPopup, "shown");
+}
+
+function popupHidden(aPopup) {
+  return BrowserTestUtils.waitForPopupEvent(aPopup, "hidden");
+}
+
+// This is a simpler version of the context menu check that
+// exists in contextmenu_common.js.
+function checkContextMenu(aContextMenu, aExpectedEntries, aWindow = window) {
+  let children = [...aContextMenu.children];
+  // Ignore hidden nodes:
+  children = children.filter(n => !n.hidden);
+
+  for (let i = 0; i < children.length; i++) {
+    let menuitem = children[i];
+    try {
+      if (aExpectedEntries[i][0] == "---") {
+        is(menuitem.localName, "menuseparator", "menuseparator expected");
+        continue;
+      }
+
+      let selector = aExpectedEntries[i][0];
+      ok(
+        menuitem.matches(selector),
+        "menuitem should match " + selector + " selector"
+      );
+      let commandValue = menuitem.getAttribute("command");
+      let relatedCommand = commandValue
+        ? aWindow.document.getElementById(commandValue)
+        : null;
+      let menuItemDisabled = relatedCommand
+        ? relatedCommand.getAttribute("disabled") == "true"
+        : menuitem.getAttribute("disabled") == "true";
+      is(
+        menuItemDisabled,
+        !aExpectedEntries[i][1],
+        "disabled state for " + selector
+      );
+    } catch (e) {
+      ok(false, "Exception when checking context menu: " + e);
+    }
+  }
+}
+
+function waitForOverflowButtonShown(win = window) {
+  info("Waiting for overflow button to show");
+  let ov = win.document.getElementById("nav-bar-overflow-button");
+  return waitForElementShown(ov.icon);
+}
+function waitForElementShown(element) {
+  return BrowserTestUtils.waitForCondition(() => {
+    info("Checking if element has non-0 size");
+    // We intentionally flush layout to ensure the element is actually shown.
+    let rect = element.getBoundingClientRect();
+    return rect.width > 0 && rect.height > 0;
+  });
+}
+
+/**
+ * Opens the history panel through the history toolbarbutton in the
+ * navbar and returns a promise that resolves as soon as the panel is open
+ * is showing.
+ */
+async function openHistoryPanel(doc = document) {
+  await waitForOverflowButtonShown();
+  await doc.getElementById("nav-bar").overflowable.show();
+  info("Menu panel was opened");
+
+  let historyButton = doc.getElementById("history-panelmenu");
+  Assert.ok(historyButton, "History button appears in Panel Menu");
+
+  historyButton.click();
+
+  let historyPanel = doc.getElementById("PanelUI-history");
+  return BrowserTestUtils.waitForEvent(historyPanel, "ViewShown");
+}
+
+/**
+ * Closes the history panel and returns a promise that resolves as sooon
+ * as the panel is closed.
+ */
+async function hideHistoryPanel(doc = document) {
+  let historyView = doc.getElementById("PanelUI-history");
+  let historyPanel = historyView.closest("panel");
+  let promise = BrowserTestUtils.waitForEvent(historyPanel, "popuphidden");
+  historyPanel.hidePopup();
+  return promise;
+}
diff --git a/browser/components/customizableui/test/support/test_967000_charEncoding_page.html b/browser/components/customizableui/test/support/test_967000_charEncoding_page.html
new file mode 100644
index 0000000000..7932b16f12
--- /dev/null
+++ b/browser/components/customizableui/test/support/test_967000_charEncoding_page.html
@@ -0,0 +1,11 @@
+<!DOCTYPE html>
+<html>
+  <head>
+    <meta charset="iso-8859-1">
+    <title>Test page</title>
+  </head>
+
+  <body>
+  This is a test page
+  </body>
+</html>
diff --git a/browser/components/customizableui/test/unit/test_unified_extensions_migration.js b/browser/components/customizableui/test/unit/test_unified_extensions_migration.js
new file mode 100644
index 0000000000..b692c1d76f
--- /dev/null
+++ b/browser/components/customizableui/test/unit/test_unified_extensions_migration.js
@@ -0,0 +1,368 @@
+/* Any copyright is dedicated to the Public Domain.
+http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+// We're in an xpcshell test but have an eslint browser test env applied;
+// We definitely do need to manually import CustomizableUI.
+// eslint-disable-next-line mozilla/no-redeclare-with-import-autofix
+const { CustomizableUI } = ChromeUtils.importESModule(
+  "resource:///modules/CustomizableUI.sys.mjs"
+);
+
+do_get_profile();
+
+// Make Cu.isInAutomation true. This is necessary so that we can use
+// CustomizableUIInternal.
+Services.prefs.setBoolPref(
+  "security.turn_off_all_security_so_that_viruses_can_take_over_this_computer",
+  true
+);
+
+const CustomizableUIInternal = CustomizableUI.getTestOnlyInternalProp(
+  "CustomizableUIInternal"
+);
+
+// Migration 19 was the Unified Extensions migration version introduced
+// in 109, so we'll run tests by artificially setting the migration version
+// to one value earlier.
+const PRIOR_MIGRATION_VERSION = 18;
+
+/**
+ * Writes customization state into CustomizableUI and then performs the forward migration
+ * for Unified Extensions.
+ *
+ * @param {object|null} stateObj An object that will be structure-cloned and
+ *   written into CustomizableUI's internal `gSavedState` state variable. Should
+ *   not include the currentVersion property, as this will be set automatically by
+ *   function if stateObj is not null.
+ * @returns {object}
+ *   the saved state object (minus the currentVersion property).
+ */
+function migrateForward(stateObj) {
+  // We make sure to use structuredClone here so that we don't end up comparing
+  // SAVED_STATE against itself.
+  let stateToSave = structuredClone(stateObj);
+  if (stateToSave) {
+    stateToSave.currentVersion = PRIOR_MIGRATION_VERSION;
+  }
+
+  CustomizableUI.setTestOnlyInternalProp("gSavedState", stateToSave);
+  CustomizableUIInternal._updateForNewVersion();
+
+  let migratedState = CustomizableUI.getTestOnlyInternalProp("gSavedState");
+  if (migratedState) {
+    delete migratedState.currentVersion;
+  }
+  return migratedState;
+}
+
+/**
+ * Test that attempting a migration on a new profile with no saved
+ * state exits safely.
+ */
+add_task(async function test_no_saved_state() {
+  let migratedState = migrateForward(null);
+
+  Assert.deepEqual(
+    migratedState,
+    null,
+    "gSavedState should not have been modified"
+  );
+});
+
+/**
+ * Test that attempting a migration on a new profile with no saved
+ * state exits safely.
+ */
+add_task(async function test_no_saved_placements() {
+  let migratedState = migrateForward({});
+
+  Assert.deepEqual(
+    migratedState,
+    {},
+    "gSavedState should not have been modified"
+  );
+});
+
+/**
+ * Test that placements that don't involve any extension buttons are
+ * not changed during the migration.
+ */
+add_task(async function test_no_extensions() {
+  const SAVED_STATE = {
+    placements: {
+      "nav-bar": [
+        "back-button",
+        "forward-button",
+        "spring",
+        "urlbar-container",
+        "save-to-pocket-button",
+      ],
+      "toolbar-menubar": [
+        "home-button",
+        "menubar-items",
+        "spring",
+        "downloads-button",
+      ],
+      TabsToolbar: [
+        "firefox-view-button",
+        "tabbrowser-tabs",
+        "new-tab-button",
+        "alltabs-button",
+        "developer-button",
+      ],
+      PersonalToolbar: ["personal-bookmarks", "fxa-toolbar-menu-button"],
+      "widget-overflow-fixed-list": ["privatebrowsing-button", "panic-button"],
+    },
+  };
+
+  // ADDONS_AREA should end up with an empty array as its set of placements.
+  const EXPECTED_STATE = structuredClone(SAVED_STATE);
+  EXPECTED_STATE.placements[CustomizableUI.AREA_ADDONS] = [];
+
+  let migratedState = migrateForward(SAVED_STATE);
+
+  Assert.deepEqual(
+    migratedState,
+    EXPECTED_STATE,
+    "Got the expected state after the migration."
+  );
+});
+
+/**
+ * Test that if there's an existing set of items in CustomizableUI.AREA_ADDONS,
+ * and no extension buttons to migrate from the overflow menu, then we don't
+ * change the state at all.
+ */
+add_task(async function test_existing_browser_actions_no_movement() {
+  const SAVED_STATE = {
+    placements: {
+      "nav-bar": [
+        "back-button",
+        "forward-button",
+        "spring",
+        "urlbar-container",
+        "save-to-pocket-button",
+      ],
+      "toolbar-menubar": [
+        "home-button",
+        "menubar-items",
+        "spring",
+        "downloads-button",
+      ],
+      TabsToolbar: [
+        "firefox-view-button",
+        "tabbrowser-tabs",
+        "new-tab-button",
+        "alltabs-button",
+        "developer-button",
+      ],
+      PersonalToolbar: ["personal-bookmarks", "fxa-toolbar-menu-button"],
+      "widget-overflow-fixed-list": ["privatebrowsing-button", "panic-button"],
+      "unified-extensions-area": ["ext0-browser-action", "ext1-browser-action"],
+    },
+  };
+
+  let migratedState = migrateForward(SAVED_STATE);
+
+  Assert.deepEqual(
+    migratedState,
+    SAVED_STATE,
+    "The saved state should not have changed after migration."
+  );
+});
+
+/**
+ * Test that we can migrate extension buttons out from the overflow panel
+ * into the addons panel.
+ */
+add_task(async function test_migrate_extension_buttons() {
+  const SAVED_STATE = {
+    placements: {
+      "nav-bar": [
+        "back-button",
+        "forward-button",
+        "spring",
+        "urlbar-container",
+        "save-to-pocket-button",
+      ],
+      "toolbar-menubar": [
+        "home-button",
+        "menubar-items",
+        "spring",
+        "downloads-button",
+      ],
+      TabsToolbar: [
+        "firefox-view-button",
+        "tabbrowser-tabs",
+        "new-tab-button",
+        "alltabs-button",
+        "developer-button",
+      ],
+      PersonalToolbar: ["personal-bookmarks", "fxa-toolbar-menu-button"],
+      "widget-overflow-fixed-list": [
+        "ext0-browser-action",
+        "privatebrowsing-button",
+        "ext1-browser-action",
+        "panic-button",
+        "ext2-browser-action",
+      ],
+    },
+  };
+  const EXPECTED_STATE = structuredClone(SAVED_STATE);
+  EXPECTED_STATE.placements[CustomizableUI.AREA_FIXED_OVERFLOW_PANEL] = [
+    "privatebrowsing-button",
+    "panic-button",
+  ];
+  EXPECTED_STATE.placements[CustomizableUI.AREA_ADDONS] = [
+    "ext0-browser-action",
+    "ext1-browser-action",
+    "ext2-browser-action",
+  ];
+
+  let migratedState = migrateForward(SAVED_STATE);
+
+  Assert.deepEqual(
+    migratedState,
+    EXPECTED_STATE,
+    "The saved state should not have changed after migration."
+  );
+});
+
+/**
+ * Test that we won't overwrite existing placements within the addons panel
+ * if we migrate things over from the overflow panel. We'll prepend the
+ * migrated items to the addons panel instead.
+ */
+add_task(async function test_migrate_extension_buttons_no_overwrite() {
+  const SAVED_STATE = {
+    placements: {
+      "nav-bar": [
+        "back-button",
+        "forward-button",
+        "spring",
+        "urlbar-container",
+        "save-to-pocket-button",
+      ],
+      "toolbar-menubar": [
+        "home-button",
+        "menubar-items",
+        "spring",
+        "downloads-button",
+      ],
+      TabsToolbar: [
+        "firefox-view-button",
+        "tabbrowser-tabs",
+        "new-tab-button",
+        "alltabs-button",
+        "developer-button",
+      ],
+      PersonalToolbar: ["personal-bookmarks", "fxa-toolbar-menu-button"],
+      "widget-overflow-fixed-list": [
+        "ext0-browser-action",
+        "privatebrowsing-button",
+        "ext1-browser-action",
+        "panic-button",
+        "ext2-browser-action",
+      ],
+      "unified-extensions-area": ["ext3-browser-action", "ext4-browser-action"],
+    },
+  };
+  const EXPECTED_STATE = structuredClone(SAVED_STATE);
+  EXPECTED_STATE.placements[CustomizableUI.AREA_FIXED_OVERFLOW_PANEL] = [
+    "privatebrowsing-button",
+    "panic-button",
+  ];
+  EXPECTED_STATE.placements[CustomizableUI.AREA_ADDONS] = [
+    "ext0-browser-action",
+    "ext1-browser-action",
+    "ext2-browser-action",
+    "ext3-browser-action",
+    "ext4-browser-action",
+  ];
+
+  let migratedState = migrateForward(SAVED_STATE);
+
+  Assert.deepEqual(
+    migratedState,
+    EXPECTED_STATE,
+    "The saved state should not have changed after migration."
+  );
+});
+
+/**
+ * Test that extension buttons from areas other than the overflow panel
+ * won't be moved.
+ */
+add_task(async function test_migrate_extension_buttons_elsewhere() {
+  const SAVED_STATE = {
+    placements: {
+      "nav-bar": [
+        "back-button",
+        "ext0-browser-action",
+        "forward-button",
+        "ext1-browser-action",
+        "spring",
+        "ext2-browser-action",
+        "urlbar-container",
+        "ext3-browser-action",
+        "save-to-pocket-button",
+        "ext4-browser-action",
+      ],
+      "toolbar-menubar": [
+        "home-button",
+        "ext5-browser-action",
+        "menubar-items",
+        "ext6-browser-action",
+        "spring",
+        "ext7-browser-action",
+        "downloads-button",
+        "ext8-browser-action",
+      ],
+      TabsToolbar: [
+        "firefox-view-button",
+        "ext9-browser-action",
+        "tabbrowser-tabs",
+        "ext10-browser-action",
+        "new-tab-button",
+        "ext11-browser-action",
+        "alltabs-button",
+        "ext12-browser-action",
+        "developer-button",
+        "ext13-browser-action",
+      ],
+      PersonalToolbar: [
+        "personal-bookmarks",
+        "ext14-browser-action",
+        "fxa-toolbar-menu-button",
+        "ext15-browser-action",
+      ],
+      "widget-overflow-fixed-list": [
+        "ext16-browser-action",
+        "privatebrowsing-button",
+        "ext17-browser-action",
+        "panic-button",
+        "ext18-browser-action",
+      ],
+    },
+  };
+  const EXPECTED_STATE = structuredClone(SAVED_STATE);
+  EXPECTED_STATE.placements[CustomizableUI.AREA_FIXED_OVERFLOW_PANEL] = [
+    "privatebrowsing-button",
+    "panic-button",
+  ];
+  EXPECTED_STATE.placements[CustomizableUI.AREA_ADDONS] = [
+    "ext16-browser-action",
+    "ext17-browser-action",
+    "ext18-browser-action",
+  ];
+
+  let migratedState = migrateForward(SAVED_STATE);
+
+  Assert.deepEqual(
+    migratedState,
+    EXPECTED_STATE,
+    "The saved state should not have changed after migration."
+  );
+});
diff --git a/browser/components/customizableui/test/unit/xpcshell.ini b/browser/components/customizableui/test/unit/xpcshell.ini
new file mode 100644
index 0000000000..ec14295805
--- /dev/null
+++ b/browser/components/customizableui/test/unit/xpcshell.ini
@@ -0,0 +1,6 @@
+[DEFAULT]
+head =
+skip-if = toolkit == 'android' # bug 1730213
+firefox-appdir = browser
+
+[test_unified_extensions_migration.js]