Bug 1992036 - fix broken notification for hiding navtoolbox, a=RyanVM DONTBUILD

Original Revision: https://phabricator.services.mozilla.com/D267402

Differential Revision: https://phabricator.services.mozilla.com/D267837

Author: gijsk

browser/base/content/browser-fullScreenAndPointerLock.js

@@ -947,7 +947,11 @@ var FullScreen = {
     gNavToolbox.style.marginTop =
       -gNavToolbox.getBoundingClientRect().height + "px";
     this._isChromeCollapsed = true;
-    Services.obs.notifyObservers(null, "fullscreen-nav-toolbox", "hidden");
+    Services.obs.notifyObservers(
+      gNavToolbox,
+      "fullscreen-nav-toolbox",
+      "hidden"
+    );
 
     MousePosTracker.removeListener(this);
   },