So I do some web development and there are instances where I need to test on multiple browsers (and multiple versions of browsers). It was pretty easy to install multiple instances of Firefox on my Mac (running OS X Leopard):

1. Go to Mozilla and download the dmg.
2. Mount the dmg file
3. Instead of dragging and dropping the version into your applications from the mounted image, go to your applications folder and create a new folder for the version you want.
4. Drag the application into your newly created folder
5. Drag the app from your folder into the dock (or run the app and choose to keep in dock).

Voila. You’ll be able to test sites on different versions of Firefox.  Have fun.

UPDATE: One more step I forgot about to make your life easier. Go here and learn how to create a profile for each of your accounts (like I did below) and then you won’t be prompted for updates each time you open that version of Firefox. I just saved mine into the version folder for each version.