Finally figured out how to run multiple versions of Firefox simultaneously.
First, you need to create some profiles, one for each Firefox version you want to run. Run
On Mac that's usually:
Doesn't matter which version you use to open the profile manager because the profiles are shared between all the versions, and so you only have to create the profiles once. I called my profiles: FF4, FF3, and FF2 respectively.
I installed my different versions under these locations
Once you have the profiles created, you can launch each version like such:
/Applications/Firefox4b.app/Contents/MacOS/firefox-bin -P FF4 -no-remote &
for Firefox 4 beta,
/Applications/Firefox3.app/Contents/MacOS/firefox-bin -P FF3 -no-remote &
for Firefox 3, and
/Applications/Firefox2.app/Contents/MacOS/firefox-bin -P FF2 -no-remote &
for Firefox 2, just cuz I can.
Now of course, that's quite a lot of typing to have to do each time you want to launch the browser so I put those one-liners into handy little shell scripts which you can do too.
Happy happy joy joy!
For other platforms it's more or less the same thing, just the installation paths would be different.