Dave Martorana’s MultiFireFox, lets you run Firefox 2 and Firefox 3 beta side-by-side on Mac OS X without wiping or messing up your stable Firefox 2 installation profile. You can hit the command line, create a new profile and then customize the shortcut to each version of Firefox so that it launches the correct profile (or manually choose what profile you want at the start of each Firefox session), but that can be a pain -- especially if you want to try out different plugin configurations in Firefox 3.
You can download the dmg, or the zip of the source if you want to dig around to port this to Windows and not only will you get Firefox 3 beta 3, but a very tidy little launching application. Upon running MultiFireFox for the first time, you will be instructed to create a second profile if you do not already have one (and the profile manager will start up automatically, no going into the command line to get it started). From that point, you can choose what profile and what version of Firefox you want to launch. You can even run the two side-by-side to test performance.
MultiFireFox, Firefox, Mac OS X, Software, Freeware, Download, Utility