Having recently switched to using Firefox and having always complained about Microsoft's PIA move by making it so difficult to run multiple version of IE at one, I was suprised to find that FireFox has the same stupid issue. Fire fox uses the same default non segrated profile path for every version.

The Fix (same idea for win and mac I believe):

Run c:\\{your Firefox path}\\firefox -profilemanager -no-remote

Create a 2 new profiles, I called mine FireFox3Rc1 and Firefox2. Use alpha-numeric and stay away from spaces. Once you've created them go ahead and exit.

Now you must modify the shortcuts for firefox, respectfully.

c:\\{FF2 Path}firefox.exe -P FireFox3 -no-remote
c:\\{FF3 Path}firefox.exe -P FireFox3RC1 -no-remote

Technically there is no need to change profiles for both FF version. But I'm assuming I'll even have a 3rd version here of FF3 soon and I hate using default profile paths...

Btw: If you need to run multiple version of IE, I have been using Multiple IE for some time with great success.