diff -r 000000000000 -r 6474c204b198 toolkit/modules/PrivateBrowsingUtils.jsm
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/toolkit/modules/PrivateBrowsingUtils.jsm	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,67 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this file,
+ * You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+this.EXPORTED_SYMBOLS = ["PrivateBrowsingUtils"];
+
+Components.utils.import("resource://gre/modules/Services.jsm");
+
+const kAutoStartPref = "browser.privatebrowsing.autostart";
+
+// This will be set to true when the PB mode is autostarted from the command
+// line for the current session.
+let gTemporaryAutoStartMode = false;
+
+const Cc = Components.classes;
+const Ci = Components.interfaces;
+
+this.PrivateBrowsingUtils = {
+  isWindowPrivate: function pbu_isWindowPrivate(aWindow) {
+    return this.privacyContextFromWindow(aWindow).usePrivateBrowsing;
+  },
+
+  privacyContextFromWindow: function pbu_privacyContextFromWindow(aWindow) {
+    return aWindow.QueryInterface(Ci.nsIInterfaceRequestor)
+                  .getInterface(Ci.nsIWebNavigation)
+                  .QueryInterface(Ci.nsILoadContext);
+  },
+
+  get permanentPrivateBrowsing() {
+    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;
+  },
+
+  whenHiddenPrivateWindowReady: function pbu_whenHiddenPrivateWindowReady(cb) {
+    Components.utils.import("resource://gre/modules/Timer.jsm");
+
+    let win = Services.appShell.hiddenPrivateDOMWindow;
+    function isNotLoaded() {
+      return ["complete", "interactive"].indexOf(win.document.readyState) == -1;
+    }
+    if (isNotLoaded()) {
+      setTimeout(function poll() {
+        if (isNotLoaded()) {
+          setTimeout(poll, 100);
+          return;
+        }
+        cb(Services.appShell.hiddenPrivateDOMWindow);
+      }, 4);
+    } else {
+      cb(Services.appShell.hiddenPrivateDOMWindow);
+    }
+  }
+};
+