Good news testers: MultiFirefox is a free application that runs multiple versions of Firefox at the same time.
There are several reasons why you would want to run multiple instances of Firefox. For one, web developers still have to test their web applications for users of Firefox 2. Now they'll have to update for Firefox 3 too – Hooray for progress. Also, Firefox 3 is brand-spankin-new, which means a lot of the heavy-weight web sites may not have support for the newest state-of-the-art browser. Having Firefox 2 around allows you to open those old websites up without having to close your open windows and shut down your primary browser.
Out of the box, Firefox doesn't allow you to run multiple versions of the browser because the instances share much of the same profile data.
There is a manual way to get around it by starting up Firefox from the command line with some arguments (By the way, this command works in Linux and Windows as well):
firefox -P -no-remote
However, it's not very intuitive from Mac OS because you have to enter Terminal to do it. MultiFirefox performs pretty much the same function but through a swift and easy to use application download.
