Mozilla Firefox Collection: Easy Website Testing On Multiple Versions of Firefox
The Edskes Software Mozilla Firefox Collection (ESMFC) contains multiple versions of the Mozilla Firefox, all of which are standalone so they can be used at the same time. The Mozilla Firefox Collection lets you see how your websites look when rendered using different versions of the Gecko render engine. The Firefox collection comes from the same person who prepared the Internet Explorer collection kit containing standalone executables of different IE versions.
Link: Final Builds Site - Edskes Software Mozilla Firefox Collection