With Firefox 4 just around the corner it is time to get those extensions upgraded, flash dumped and sites tested. Quite a few developers will to be testing in stable 3.6 as well as the new betas. In fact at this very moment I'm working on porting a Google Chrome extension to Firefox using the new Add-on SDK . Luckily Firefox makes it easy to run multiple version of concurrently. Keep reading to figure learn how. 1) First download the Firefox 4 beta (4.0b8 at the time of writing), open the dmg, copy Firefox.app into your Applications directory and rename it to Firefox4.0b8.app. Then download Firefox 3 (3.6.13 at the time of writing), open the dmg, copy Firefox.app into the Applications folder and rename it to Firefox3.6.app. 2) You now have two version of Firefox installed but if you open them at the same time all kinds of conflicts with sessions and dirty exits will cause issues. We can fix that by creating multiple user profiles so that each Fx instance has it's own sa...