Bug 1717612 - Disable window.sidebar from early-beta-or-earlier r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D118472

dom/webidl/Window.webidl

@@ -573,7 +573,7 @@ partial interface Window {
 // Mozilla extension
 // Sidebar is deprecated and it will be removed in the next cycles. See bug 1640138.
 partial interface Window {
-  [Replaceable, Throws, UseCounter]
+  [Replaceable, Throws, UseCounter, Pref="dom.window.sidebar.enabled"]
   readonly attribute (External or WindowProxy) sidebar;
 };
 #endif

modules/libpref/init/StaticPrefList.yaml

@@ -3715,6 +3715,11 @@
   value: true
   mirror: always
 
+- name: dom.window.sidebar.enabled
+  type: bool
+  value: @IS_NOT_EARLY_BETA_OR_EARLIER@
+  mirror: always
+
 - name: dom.worker.canceling.timeoutMilliseconds
   type: RelaxedAtomicUint32
   value: 30000    # 30 seconds