--- a/toolkit/content/PrivateBrowsingUtils.jsm
+++ b/toolkit/content/PrivateBrowsingUtils.jsm
@@ -22,18 +22,23 @@ this.PrivateBrowsingUtils = {
 
   privacyContextFromWindow: function pbu_privacyContextFromWindow(aWindow) {
     return aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
                   .getInterface(Ci.nsIWebNavigation)
                   .QueryInterface(Ci.nsILoadContext);
   },
 
   get permanentPrivateBrowsing() {
-    return gTemporaryAutoStartMode ||
-           Services.prefs.getBoolPref(kAutoStartPref, false);
+    try {
+      return gTemporaryAutoStartMode ||
+             Services.prefs.getBoolPref(kAutoStartPref);
+    } catch (e) {
+      // The pref does not exist
+      return false;
+    }
   },
 
   // These should only be used from internal code
   enterTemporaryAutoStartMode: function pbu_enterTemporaryAutoStartMode() {
     gTemporaryAutoStartMode = true;
   },
   get isInTemporaryAutoStartMode() {
     return gTemporaryAutoStartMode;