code: function privateBrowsing(controller) {
    this._controller = controller;
    this._handler = null;
  
    /**
     * Menu item in the main menu to enter/leave Private Browsing mode
     * @private
     */
    this._pbMenuItem = new elementslib.Elem(this._controller.menus['tools-menu'].privateBrowsingItem);
    this._pbTransitionItem = new elementslib.ID(this._controller.window.document, "Tools:PrivateBrowsing");
  
    this.__defineGetter__('_pbs', function() {
      delete this._pbs;
      return this._pbs = Cc["@mozilla.org/privatebrowsing;1"].
                         getService(Ci.nsIPrivateBrowsingService);
    });
  }
docs: Create a new privateBrowsing instance.
  
  @class This class adds support for the Private Browsing mode
  @param {MozMillController} controller
  MozMillController to use for the modal entry dialog