--- a/browser/modules/BrowserUITelemetry.jsm
+++ b/browser/modules/BrowserUITelemetry.jsm
@@ -38,16 +38,17 @@ XPCOMUtils.defineLazyGetter(this, "DEFAU
       "privatebrowsing-button",
       "save-page-button",
       "print-button",
       "history-panelmenu",
       "fullscreen-button",
       "find-button",
       "preferences-button",
       "add-ons-button",
+      "sync-button",
       "developer-button",
     ],
     "nav-bar": [
       "urlbar-container",
       "search-container",
       "bookmarks-menu-button",
       "pocket-button",
       "downloads-button",
@@ -86,17 +87,16 @@ XPCOMUtils.defineLazyGetter(this, "DEFAU
 });
 
 XPCOMUtils.defineLazyGetter(this, "PALETTE_ITEMS", function() {
   let result = [
     "open-file-button",
     "developer-button",
     "feed-button",
     "email-link-button",
-    "sync-button",
   ];
 
   let panelPlacements = DEFAULT_AREA_PLACEMENTS["PanelUI-contents"];
   if (panelPlacements.indexOf("characterencoding-button") == -1) {
     result.push("characterencoding-button");
   }
 
   if (Services.prefs.getBoolPref("privacy.panicButton.enabled")) {

--- a/browser/modules/test/browser.ini
+++ b/browser/modules/test/browser.ini
@@ -1,13 +1,14 @@
 [DEFAULT]
 support-files =
   head.js
 
 [browser_BrowserUITelemetry_buckets.js]
+[browser_BrowserUITelemetry_defaults.js]
 [browser_ProcessHangNotifications.js]
 skip-if = !e10s
 [browser_ContentSearch.js]
 support-files =
   contentSearch.js
   contentSearchBadImage.xml
   contentSearchSuggestions.sjs
   contentSearchSuggestions.xml

new file mode 100644
--- /dev/null
+++ b/browser/modules/test/browser_BrowserUITelemetry_defaults.js
@@ -0,0 +1,31 @@
+// The purpose of this test is to ensure that by default, BrowserUITelemetry
+// isn't reporting any UI customizations. This is primarily so changes to
+// customizableUI (eg, new buttons, button location changes) also have a
+// corresponding BrowserUITelemetry change.
+
+function test() {
+  let s = {};
+  Cu.import("resource:///modules/CustomizableUI.jsm", s);
+  Cu.import("resource:///modules/BrowserUITelemetry.jsm", s);
+
+  let { CustomizableUI, BrowserUITelemetry } = s;
+
+  Assert.ok(CustomizableUI.inDefaultState,
+            "No other test should have left CUI in a dirty state.");
+
+  let result = BrowserUITelemetry._getWindowMeasurements(window, 0);
+
+  Assert.deepEqual(result.defaultMoved, []);
+  Assert.deepEqual(result.nondefaultAdded, []);
+  // This one is a bit weird - the "social-share-button" is dynamically added
+  // to the toolbar as the feature is first used - but it's listed as being in
+  // the toolbar by default so it doesn't end up in nondefaultAdded once it
+  // is created. The end result is that it ends up in defaultRemoved before
+  // the feature has been activated.
+  // Bug 1273358 exists to fix this.
+  Assert.deepEqual(result.defaultRemoved, ["social-share-button"]);
+
+  // And mochi insists there's only a single window with a single tab when
+  // starting a test, so check that for good measure.
+  Assert.deepEqual(result.visibleTabs, [1]);
+}