--- a/browser/components/privatebrowsing/test/browser/Makefile.in
+++ b/browser/components/privatebrowsing/test/browser/Makefile.in
@@ -16,16 +16,17 @@ MOCHITEST_BROWSER_FILES =  \
 		browser_privatebrowsing_certexceptionsui.js \
 		browser_privatebrowsing_clearplugindata.js \
 		browser_privatebrowsing_clearplugindata.html \
 		browser_privatebrowsing_commandline_toggle.js \
 		browser_privatebrowsing_concurrent.js \
 		browser_privatebrowsing_concurrent_page.html \
 		browser_privatebrowsing_crh.js \
 		browser_privatebrowsing_downloadLastDir.js \
+		browser_privatebrowsing_downloadLastDir_c.js \
 		browser_privatebrowsing_fastswitch.js \
 		browser_privatebrowsing_findbar.js \
 		browser_privatebrowsing_forgetthissite.js \
 		browser_privatebrowsing_geoprompt.js \
 		browser_privatebrowsing_geoprompt_page.html \
 		browser_privatebrowsing_import.js \
 		browser_privatebrowsing_lastpbcontextexited.js \
 		browser_privatebrowsing_localStorage.js \

new file mode 100644
--- /dev/null
+++ b/browser/components/privatebrowsing/test/browser/browser_privatebrowsing_downloadLastDir_c.js
@@ -0,0 +1,111 @@
+/* -*- Mode: javascript; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
+/* 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/. */
+
+Cu.import("resource://gre/modules/DownloadLastDir.jsm");
+Cu.import("resource://gre/modules/FileUtils.jsm");
+Cu.import("resource://gre/modules/Services.jsm");
+Cu.import("resource://mochikit/MockFilePicker.jsm");
+
+let pb = Cc["@mozilla.org/privatebrowsing;1"].
+         getService(Ci.nsIPrivateBrowsingService);
+
+MockFilePicker.init();
+MockFilePicker.returnValue = Ci.nsIFilePicker.returnOK;
+
+function test() {
+  let stringBundleToRestore = ContentAreaUtils.stringBundle;
+  let validateFileNameToRestore = validateFileName;
+
+  Services.prefs.setBoolPref("browser.privatebrowsing.keep_current_session", true);
+  let prefs = Services.prefs.getBranch("browser.download.");
+  let tmpDir = FileUtils.getDir("TmpD", [], true);
+  function newDirectory() {
+    let dir = tmpDir.clone();
+    dir.append("testdir");
+    dir.createUnique(Ci.nsIFile.DIRECTORY_TYPE, 0700);
+    return dir;
+  }
+
+  function newFileInDirectory(dir) {
+    let file = dir.clone();
+    file.append("testfile");
+    file.createUnique(Ci.nsIFile.DIRECTORY_TYPE, 0600);
+    return file;
+  }
+
+  let dir1 = newDirectory();
+  let dir2 = newDirectory();
+  let dir3 = newDirectory();
+  let file1 = newFileInDirectory(dir1);
+  let file2 = newFileInDirectory(dir2);
+  let file3 = newFileInDirectory(dir3);
+
+  // cleanup function registration
+  registerCleanupFunction(function () {
+    Services.prefs.clearUserPref("browser.privatebrowsing.keep_current_session");
+    Services.prefs.clearUserPref("browser.download.lastDir");
+    [dir1, dir2, dir3].forEach(function(dir) dir.remove(true));
+    MockFilePicker.cleanup();
+    ContentAreaUtils.stringBundle = stringBundleToRestore;
+    validateFileName = validateFileNameToRestore;
+    gDownloadLastDir.cleanupPrivateFile();
+    delete FileUtils;
+    delete gDownloadLastDir;
+  });
+
+  // Overwrite stringBundle to return an object masquerading as a string bundle
+  delete ContentAreaUtils.stringBundle;
+  ContentAreaUtils.stringBundle = {
+    GetStringFromName: function() ""
+  };
+
+  // Overwrite validateFileName to validate everything
+  validateFileName = function(foo) foo;
+
+  let params = {
+    fpTitleKey: "test",
+    fileInfo: new FileInfo("test.txt", "test.txt", "test", "txt", "http://mozilla.org/test.txt"),
+    contentType: "text/plain",
+    saveMode: SAVEMODE_FILEONLY,
+    saveAsType: kSaveAsType_Complete,
+    file: null
+  };
+
+  prefs.setComplexValue("lastDir", Ci.nsIFile, tmpDir);
+  MockFilePicker.returnFiles = [file1];
+  MockFilePicker.displayDirectory = null;
+  ok(getTargetFile(params), "Show the file picker dialog with given params");
+  // file picker should start with browser.download.lastDir
+  is(MockFilePicker.displayDirectory.path, tmpDir.path, "file picker should start with browser.download.lastDir");
+  // browser.download.lastDir should be modified before entering the private browsing mode
+  is(prefs.getComplexValue("lastDir", Ci.nsIFile).path, dir1.path, "LastDir should be modified before entering PB mode");
+  // gDownloadLastDir should be usable outside of the private browsing mode
+  is(gDownloadLastDir.file.path, dir1.path, "gDownloadLastDir should be usable outside of the PB mode");
+
+  pb.privateBrowsingEnabled = true;
+  is(prefs.getComplexValue("lastDir", Ci.nsIFile).path, dir1.path, "LastDir should be that set before PB mode");
+  MockFilePicker.returnFiles = [file2];
+  MockFilePicker.displayDirectory = null;
+  ok(getTargetFile(params), "Show the file picker dialog with the given params");
+  // file picker should start with browser.download.lastDir as set before entering the private browsing mode
+  is(MockFilePicker.displayDirectory.path, dir1.path, "File picker should start with LastDir set before entering PB mode");
+  // browser.download.lastDir should not be modified inside the private browsing mode
+  is(prefs.getComplexValue("lastDir", Ci.nsIFile).path, dir1.path, "LastDir should not be modified inside PB mode");
+  // but gDownloadLastDir should be modified
+  is(gDownloadLastDir.file.path, dir2.path, "gDownloadLastDir should be modified");
+
+  pb.privateBrowsingEnabled = false;
+  // gDownloadLastDir should be cleared after leaving the private browsing mode
+  is(gDownloadLastDir.file.path, dir1.path, "gDownloadLastDir should be cleared after leaving PB mode");
+  MockFilePicker.returnFiles = [file3];
+  MockFilePicker.displayDirectory = null;
+  ok(getTargetFile(params), "Show the file picker dialog with the given params");
+  // file picker should start with browser.download.lastDir as set before entering the private browsing mode
+  is(MockFilePicker.displayDirectory.path, dir1.path, "File picker should start with LastDir set before PB mode");
+  // browser.download.lastDir should be modified after leaving the private browsing mode
+  is(prefs.getComplexValue("lastDir", Ci.nsIFile).path, dir3.path, "LastDir should be modified after leaving PB mode");
+  // gDownloadLastDir should be usable after leaving the private browsing mode
+  is(gDownloadLastDir.file.path, dir3.path, "gDownloadLastDir should be usable after leaving PB mode");
+}

deleted file mode 100644
--- a/toolkit/content/tests/unit/test_privatebrowsing_downloadLastDir_c.js
+++ /dev/null
@@ -1,130 +0,0 @@
-/* -*- Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2 -*- */
-/* 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/. */
-
-const Ci = Components.interfaces;
-const Cc = Components.classes;
-
-function loadUtilsScript() {
-  let loader = Cc["@mozilla.org/moz/jssubscript-loader;1"].
-               getService(Ci.mozIJSSubScriptLoader);
-  loader.loadSubScript("chrome://global/content/contentAreaUtils.js");
-  Components.utils.import("resource://gre/modules/DownloadLastDir.jsm");
-}
-
-do_get_profile();
-
-let window = {};
-function run_test()
-{
-  let pb;
-  try {
-    pb = Cc["@mozilla.org/privatebrowsing;1"].
-         getService(Ci.nsIPrivateBrowsingService);
-  } catch (e) {
-    print("PB service is not available, bail out");
-    return;
-  }
-
-  loadUtilsScript();
-
-  let prefsService = Cc["@mozilla.org/preferences-service;1"].
-                     getService(Ci.nsIPrefService).
-                     QueryInterface(Ci.nsIPrefBranch);
-  prefsService.setBoolPref("browser.privatebrowsing.keep_current_session", true);
-  let prefs = prefsService.getBranch("browser.download.");
-  let dirSvc = Cc["@mozilla.org/file/directory_service;1"].
-               getService(Ci.nsIProperties);
-  let tmpDir = dirSvc.get("TmpD", Ci.nsILocalFile);
-  function newDirectory() {
-    let dir = tmpDir.clone();
-    dir.append("testdir" + Math.floor(Math.random() * 10000));
-    dir.createUnique(Ci.nsIFile.DIRECTORY_TYPE, 0700);
-    return dir;
-  }
-  function newFileInDirectory(dir) {
-    let file = dir.clone();
-    file.append("testfile" + Math.floor(Math.random() * 10000));
-    file.createUnique(Ci.nsIFile.DIRECTORY_TYPE, 0600);
-    return file;
-  }
-  let dir1 = newDirectory();
-  let dir2 = newDirectory();
-  let dir3 = newDirectory();
-  let file1 = newFileInDirectory(dir1);
-  let file2 = newFileInDirectory(dir2);
-  let file3 = newFileInDirectory(dir3);
-
-  // overwrite makeFilePicker, as we don't want to create a real filepicker object
-  let fp = {
-    appendFilter: function() {},
-    appendFilters: function() {},
-    init: function() {},
-    show: function() Ci.nsIFilePicker.returnOK,
-    displayDirectory: null,
-    file: file1
-  };
-  makeFilePicker = function() fp;
-
-  // Overwrite stringBundle to return an object masquerading as a string bundle
-  delete ContentAreaUtils.stringBundle;
-  ContentAreaUtils.stringBundle = {
-    GetStringFromName: function() ""
-  };
-
-  // Overwrite validateFileName to validate everything
-  validateFileName = function(foo) foo;
-
-  let params = {
-    fpTitleKey: "test",
-    fileInfo: new FileInfo("test.txt", "test.txt", "test", "txt", "http://mozilla.org/test.txt"),
-    contentType: "text/plain",
-    saveMode: SAVEMODE_FILEONLY,
-    saveAsType: kSaveAsType_Complete,
-    file: null
-  };
-
-  prefs.setComplexValue("lastDir", Ci.nsILocalFile, tmpDir);
-
-  do_check_true(getTargetFile(params));
-  // file picker should start with browser.download.lastDir
-  do_check_eq(fp.displayDirectory.path, tmpDir.path);
-  // browser.download.lastDir should be modified before entering the private browsing mode
-  do_check_eq(prefs.getComplexValue("lastDir", Ci.nsILocalFile).path, dir1.path);
-  // gDownloadLastDir should be usable outside of the private browsing mode
-  do_check_eq(gDownloadLastDir.file.path, dir1.path);
-
-  pb.privateBrowsingEnabled = true;
-  do_check_eq(prefs.getComplexValue("lastDir", Ci.nsILocalFile).path, dir1.path);
-  fp.file = file2;
-  fp.displayDirectory = null;
-  do_check_true(getTargetFile(params));
-  // file picker should start with browser.download.lastDir as set before entering the private browsing mode
-  do_check_eq(fp.displayDirectory.path, dir1.path);
-  // browser.download.lastDir should not be modified inside the private browsing mode
-  do_check_eq(prefs.getComplexValue("lastDir", Ci.nsILocalFile).path, dir1.path);
-  // but gDownloadLastDir should be modified
-  do_check_eq(gDownloadLastDir.file.path, dir2.path);
-
-  pb.privateBrowsingEnabled = false;
-  // gDownloadLastDir should be cleared after leaving the private browsing mode
-  do_check_eq(gDownloadLastDir.file.path, dir1.path);
-  fp.file = file3;
-  fp.displayDirectory = null;
-  do_check_true(getTargetFile(params));
-  // file picker should start with browser.download.lastDir as set before entering the private browsing mode
-  do_check_eq(fp.displayDirectory.path, dir1.path);
-  // browser.download.lastDir should be modified after leaving the private browsing mode
-  do_check_eq(prefs.getComplexValue("lastDir", Ci.nsILocalFile).path, dir3.path);
-  // gDownloadLastDir should be usable after leaving the private browsing mode
-  do_check_eq(gDownloadLastDir.file.path, dir3.path);
-
-  // cleanup
-  Cc["@mozilla.org/observer-service;1"]
-    .getService(Ci.nsIObserverService)
-    .notifyObservers(null, "quit-application", null);
-
-  prefsService.clearUserPref("browser.privatebrowsing.keep_current_session");
-  [dir1, dir2, dir3].forEach(function(dir) dir.remove(true));
-}

--- a/toolkit/content/tests/unit/xpcshell.ini
+++ b/toolkit/content/tests/unit/xpcshell.ini
@@ -1,8 +1,7 @@
 [DEFAULT]
 head = 
 tail = 
 
 [test_contentAreaUtils.js]
 [test_dict.js]
-[test_privatebrowsing_downloadLastDir_c.js]
 [test_propertyListsUtils.js]