--- a/browser/components/customizableui/test/browser.ini
+++ b/browser/components/customizableui/test/browser.ini
@@ -130,16 +130,17 @@ skip-if = os == "linux"
 
 [browser_985815_propagate_setToolbarVisibility.js]
 [browser_981305_separator_insertion.js]
 [browser_988072_sidebar_events.js]
 [browser_989338_saved_placements_not_resaved.js]
 [browser_989751_subviewbutton_class.js]
 [browser_987177_destroyWidget_xul.js]
 [browser_987177_xul_wrapper_updating.js]
+[browser_987185_syncButton.js]
 [browser_987492_window_api.js]
 [browser_987640_charEncoding.js]
 [browser_992747_toggle_noncustomizable_toolbar.js]
 [browser_993322_widget_notoolbar.js]
 [browser_995164_registerArea_during_customize_mode.js]
 [browser_996364_registerArea_different_properties.js]
 [browser_996635_remove_non_widgets.js]
 [browser_1003588_no_specials_in_panel.js]

new file mode 100755
--- /dev/null
+++ b/browser/components/customizableui/test/browser_987185_syncButton.js
@@ -0,0 +1,69 @@
+/* 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 syncService = {};
+Components.utils.import("resource://services-sync/service.js", syncService);
+
+let needsSetup;
+let originalSync;
+let service = syncService.Service;
+let syncWasCalled = false;
+
+add_task(function* testSyncButtonFunctionality() {
+  info("Check Sync button functionality");
+  storeInitialValues();
+  mockFunctions();
+
+  // add the Sync button to the panel
+  CustomizableUI.addWidgetToArea("sync-button", CustomizableUI.AREA_PANEL);
+
+  // check the button's functionality
+  yield PanelUI.show();
+  info("The panel menu was opened");
+
+  let syncButton = document.getElementById("sync-button");
+  ok(syncButton, "The Sync button was added to the Panel Menu");
+  syncButton.click();
+  info("The sync button was clicked");
+
+  yield waitForCondition(() => syncWasCalled);
+});
+
+add_task(function* asyncCleanup() {
+  // reset the panel UI to the default state
+  yield resetCustomization();
+  ok(CustomizableUI.inDefaultState, "The panel UI is in default state again.");
+
+  if (isPanelUIOpen()) {
+    let panelHidePromise = promisePanelHidden(window);
+    PanelUI.hide();
+    yield panelHidePromise;
+  }
+
+  restoreValues();
+});
+
+function mockFunctions() {
+  // mock needsSetup
+  gSyncUI._needsSetup = function() false;
+
+  // mock service.errorHandler.syncAndReportErrors()
+  service.errorHandler.syncAndReportErrors = mocked_syncAndReportErrors;
+}
+
+function mocked_syncAndReportErrors() {
+  syncWasCalled = true;
+}
+
+function restoreValues() {
+  gSyncUI._needsSetup = needsSetup;
+  service.sync = originalSync;
+}
+
+function storeInitialValues() {
+  needsSetup = gSyncUI._needsSetup;
+  originalSync = service.sync;
+}