Comment 470 for bug 263435

How about the browser window itself? That's what the rest of the Unfocus JS code does:

    var wm = Components.classes["@mozilla.org/appshell/window-mediator;1"]
                    .getService(Components.interfaces.nsIWindowMediator);
    var browserWindow = wm.getMostRecentWindow("navigator:browser");

    browserWindow.focus();