In Utilu Mozilla Firefox Collection you can quickly see how your websites look when rendered using different versions of the Gecko render engine. You can specify the filenames or locations (URLs) to open in all installed versions of Mozilla Firefox. Filenames or URLs that include spaces must be enclosed in quotes ("). Multiple filenames or URLs may be used by separating them by spaces. This will result in multiple tabs being opened.
http://utilu.com/UtiluMFC/
Utilu Mozilla Firefox Collection contains multiple standalone versions of Mozilla Firefox, which can be used at the same time.