--- a/browser/base/content/browser-sync.js
+++ b/browser/base/content/browser-sync.js
@@ -85,17 +85,18 @@ var gSync = {
       // we open the window. We can avoid a repaint.
       if (state.status != UIState.STATUS_NOT_CONFIGURED) {
         this.updateAllUI(state);
       }
     }
   },
 
   init() {
-    if (this._initialized) {
+    // Bail out if we're already initialized or for hidden windows.
+    if (this._initialized || window.location.href != getBrowserURL()) {
       return;
     }
 
     for (let topic of this._obs) {
       Services.obs.addObserver(this, topic, true);
     }
 
     this._generateNodeGetters();