I like the Aptana Browser Preview windo for this.
You can get it from their update site: http://download.aptana.org/tools/studio/plugin/update/studio/
After you install the Aptana plugin, open an Aptana project and there should be an option under help to install aptana features. under other you will find a Firefox/XUL browser. There may be a more direct way to install the XUL plugin, but the above procedure works.