--- a/browser/components/sessionstore/content/content-sessionStore.js
+++ b/browser/components/sessionstore/content/content-sessionStore.js
@@ -705,17 +705,17 @@ var MessageQueue = {
     Services.prefs.addObserver(TIMEOUT_DISABLED_PREF, this, false);
   },
 
   uninit() {
     Services.prefs.removeObserver(TIMEOUT_DISABLED_PREF, this);
   },
 
   observe(subject, topic, data) {
-    if (topic == TIMEOUT_DISABLED_PREF) {
+    if (topic == "nsPref:changed" && data == TIMEOUT_DISABLED_PREF) {
       this.timeoutDisabled =
         Services.prefs.getBoolPref(TIMEOUT_DISABLED_PREF);
     }
   },
 
   /**
    * Pushes a given |value| onto the queue. The given |key| represents the type
    * of data that is stored and can override data that has been queued before