--- a/browser/components/nsBrowserGlue.js
+++ b/browser/components/nsBrowserGlue.js
@@ -48,16 +48,19 @@ XPCOMUtils.defineLazyModuleGetter(this, 
                                   "resource:///modules/webrtcUI.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "PrivateBrowsingUtils",
                                   "resource://gre/modules/PrivateBrowsingUtils.jsm");
 
 XPCOMUtils.defineLazyModuleGetter(this, "KeywordURLResetPrompter",
                                   "resource:///modules/KeywordURLResetPrompter.jsm");
 
+XPCOMUtils.defineLazyModuleGetter(this, "RecentWindow",
+                                  "resource:///modules/RecentWindow.jsm");
+
 const PREF_PLUGINS_NOTIFYUSER = "plugins.update.notifyUser";
 const PREF_PLUGINS_UPDATEURL  = "plugins.update.url";
 
 // We try to backup bookmarks at idle times, to avoid doing that at shutdown.
 // Number of idle seconds before trying to backup bookmarks.  15 minutes.
 const BOOKMARKS_BACKUP_IDLE_TIME = 15 * 60;
 // Minimum interval in milliseconds between backups.
 const BOOKMARKS_BACKUP_INTERVAL = 86400 * 1000;
@@ -1579,51 +1582,19 @@ BrowserGlue.prototype = {
       Components.utils.reportError(ex);
     }
     finally {
       Services.prefs.setIntPref(SMART_BOOKMARKS_PREF, SMART_BOOKMARKS_VERSION);
       Services.prefs.savePrefFile(null);
     }
   },
 
-#ifndef XP_WIN
-#define BROKEN_WM_Z_ORDER
-#endif
-
   // this returns the most recent non-popup browser window
   getMostRecentBrowserWindow: function BG_getMostRecentBrowserWindow() {
-    function isFullBrowserWindow(win) {
-      return !win.closed &&
-             win.toolbar.visible;
-    }
-
-#ifdef BROKEN_WM_Z_ORDER
-    var win = Services.wm.getMostRecentWindow("navigator:browser");
-
-    // if we're lucky, this isn't a popup, and we can just return this
-    if (win && !isFullBrowserWindow(win)) {
-      win = null;
-      let windowList = Services.wm.getEnumerator("navigator:browser");
-      // this is oldest to newest, so this gets a bit ugly
-      while (windowList.hasMoreElements()) {
-        let nextWin = windowList.getNext();
-        if (isFullBrowserWindow(nextWin))
-          win = nextWin;
-      }
-    }
-    return win;
-#else
-    var windowList = Services.wm.getZOrderDOMWindowEnumerator("navigator:browser", true);
-    while (windowList.hasMoreElements()) {
-      let win = windowList.getNext();
-      if (isFullBrowserWindow(win))
-        return win;
-    }
-    return null;
-#endif
+    return RecentWindow.getMostRecentBrowserWindow();
   },
 
 #ifdef MOZ_SERVICES_SYNC
   /**
    * Called as an observer when Sync's "display URI" notification is fired.
    *
    * We open the received URI in a background tab.
    *

--- a/browser/modules/Makefile.in
+++ b/browser/modules/Makefile.in
@@ -23,16 +23,18 @@ EXTRA_JS_MODULES = \
 	SignInToWebsite.jsm \
 	TelemetryTimestamps.jsm \
 	Social.jsm \
 	webappsUI.jsm \
 	webrtcUI.jsm \
 	KeywordURLResetPrompter.jsm \
 	$(NULL)
 
+EXTRA_PP_JS_MODULES = RecentWindow.jsm
+
 ifeq ($(MOZ_WIDGET_TOOLKIT),windows)
 EXTRA_JS_MODULES += \
 	WindowsPreviewPerTab.jsm \
 	$(NULL)
 EXTRA_PP_JS_MODULES = \
 	WindowsJumpLists.jsm \
 	$(NULL)
 endif

new file mode 100644
--- /dev/null
+++ b/browser/modules/RecentWindow.jsm
@@ -0,0 +1,66 @@
+/* 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/. */
+
+"use strict";
+
+this.EXPORTED_SYMBOLS = ["RecentWindow"];
+
+const Cu = Components.utils;
+
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://gre/modules/PrivateBrowsingUtils.jsm");
+
+#ifndef XP_WIN
+#define BROKEN_WM_Z_ORDER
+#endif
+
+this.RecentWindow = {
+  /*
+   * Get the most recent browser window.
+   *
+   * @param aOptions an object accepting the arguments for the search.
+   *        Set the private property to true in order to restrict the
+   *        search to private windows only, or to false in order to
+   *        restrict the search to non-private windows only.  To search
+   *        in both groups, don't specify the private property.
+   */
+  getMostRecentBrowserWindow: function RW_getMostRecentBrowserWindow(aOptions) {
+    let checkPrivacy = typeof aOptions == "object" &&
+                       "private" in aOptions;
+    let wantPrivate = checkPrivacy && aOptions.private;
+
+    function isSuitableBrowserWindow(win) {
+      return (!win.closed &&
+              win.toolbar.visible &&
+              (!checkPrivacy ||
+               PrivateBrowsingUtils.isWindowPrivate(win) == wantPrivate));
+    }
+
+#ifdef BROKEN_WM_Z_ORDER
+    let win = Services.wm.getMostRecentWindow("navigator:browser");
+
+    // if we're lucky, this isn't a popup, and we can just return this
+    if (win && !isSuitableBrowserWindow(win)) {
+      win = null;
+      let windowList = Services.wm.getEnumerator("navigator:browser");
+      // this is oldest to newest, so this gets a bit ugly
+      while (windowList.hasMoreElements()) {
+        let nextWin = windowList.getNext();
+        if (isSuitableBrowserWindow(nextWin))
+          win = nextWin;
+      }
+    }
+    return win;
+#else
+    let windowList = Services.wm.getZOrderDOMWindowEnumerator("navigator:browser", true);
+    while (windowList.hasMoreElements()) {
+      let win = windowList.getNext();
+      if (isSuitableBrowserWindow(win))
+        return win;
+    }
+    return null;
+#endif
+  }
+};
+