One of the great things about virtual machines is you can test on a virtual machine, destroy it, and recreate it in pristine condition in a matter of seconds. You can do this with browsers as well. Before I mentioned PortableApps is a great thing to have on your USB drive. Well, you can also just unzip a copy to your desktop each time you want a clean version of Firefox. So keep your good install for yourself, and perhaps create another with extensions you want to test, or if you just want to test your site against an older version of Firefox. I believe it will still use plugins installed on your computer, but extensions, prefs, etc are totally separate. So each install is pristine. I keep a copy around of 1.0.x and 1.5.x as well as 2.0. So each time I want to test on an older version I can just run those. The prefs won’t mess anything up since it’s a fresh copy. You can keep your latest and greatest Firefox install with all your extensions and settings alone. This allows me to ensure everything works good in different versions without having to risk my preferences to older versions, or have to go through install/uninstall.
You can get various versions of Firefox here. Just download and extract to your desktop or some other easy to access place. I keep a copy of all the major versions around. Very handy.
I should note you can only run 1 version at a time, even though they use different preferences. Not sure if this is a fixable bug, something with serious work that just isn’t worth the effort, or something that XULRunner will take care of. Update: apparently you can, see comments below.
Unfortunately I don’t think there is a version of PortableApps for Mac/Linux users. So it may be Windows only for now.
In case you missed it, you can test IE7 via VirtualPC thanks to Microsoft.