diff --git a/services/cloudsync/CloudSyncTabs.jsm b/services/cloudsync/CloudSyncTabs.jsm
index b0921b709bc..ba9077999b0 100644
--- a/services/cloudsync/CloudSyncTabs.jsm
+++ b/services/cloudsync/CloudSyncTabs.jsm
@@ -148,8 +148,7 @@ this.Tabs = function () {
 
   let update = function (event) {
     if (event.originalTarget.linkedBrowser) {
-      let win = event.originalTarget.linkedBrowser.contentWindow;
-      if (PrivateBrowsingUtils.isWindowPrivate(win) &&
+      if (PrivateBrowsingUtils.isBrowserPrivate(event.originalTarget.linkedBrowser) &&
           !PrivateBrowsingUtils.permanentPrivateBrowsing) {
         return;
       }
diff --git a/toolkit/modules/PrivateBrowsingUtils.jsm b/toolkit/modules/PrivateBrowsingUtils.jsm
index 1c013dcaefd..847160e21b0 100644
--- a/toolkit/modules/PrivateBrowsingUtils.jsm
+++ b/toolkit/modules/PrivateBrowsingUtils.jsm
@@ -16,10 +16,16 @@ const Cc = Components.classes;
 const Ci = Components.interfaces;
 
 this.PrivateBrowsingUtils = {
+  // Rather than passing content windows to this function, please use
+  // isBrowserPrivate since it works with e10s.
   isWindowPrivate: function pbu_isWindowPrivate(aWindow) {
     return this.privacyContextFromWindow(aWindow).usePrivateBrowsing;
   },
 
+  isBrowserPrivate: function(aBrowser) {
+    return this.isWindowPrivate(aBrowser.ownerDocument.defaultView);
+  },
+
   privacyContextFromWindow: function pbu_privacyContextFromWindow(aWindow) {
     return aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
                   .getInterface(Ci.nsIWebNavigation)