One of the more useful tools that came with Internet Explorer 8 beta 1 was a toolbar button that easily switched between the beta and the previous stable iteration of the program, IE7. Nobody seems to have written a Firefox extension that does that--yet--but there are still several ways to have separate installations of Firefox 3 beta and the stable Firefox 2 on your computer, for both Mac and PC.
I covered one way Windows users can have dual installations back when FF3 beta 1 came out. Using PortableApps' Firefox 3 Portable beta 4, you can keep your Firefox 2 settings while tinkering with Firefox 3. This is especially important with the fourth beta release, as the plug-ins feature has been overhauled and many extension publishers are beginning to release FF3-compatible updates that might make your profile squeal in pain in FF2.
Another way that is more complicated involves creating a new profile in Firefox 2 before installing the FF3 beta. Once you've installed the beta, but before you launch it, you associate the shortcut for the beta executable with the new profile. It's a sound solution, but it's much easier to just grab the PortableApps version and be done with it.
Mac users aren't to be left out of this game, either. For those of you who pray at night at an altar decked out with photos of Steve Jobs and the Firefox logo, there's a simple solution to your dilemma. Download MultiFireFox, created by Dave Martorana.
This launcher application prevents the beta from wrecking your FF2 profile. Copy it and the included Firefox3.app to your Applications folder, and you'll be able to switch installations on the fly. Don't forget that you'll need to create a separate profile for the beta testing before you can try out the beta. Once you've got your profile created, update the Firefox 3 beta to the latest version. The current build of MultiFireFox comes with FF3 beta 3, not beta 4.
This solution is superior to the Windows one in that you'll be able to run both versions at the same time, switching between them at will. However, I haven't been able to get beta 4 for Mac to save my open tabs as it does in Windows.
Users who like their Web browsing to be liberally sprinkled with the flavor of the unknown can try out the nightly build of Firefox 3, code-named Minefield for a good reason: it's not for the faint-hearted.