I'm using Windows and want to use Firefox 3.5 side-by-side with the older 3.0. How can this be done?
The best way to do this is install Firefox 3.0 on your computer, then download Portable Firefox to run version 3.5.
Use Portable Firefox. It doesn't need to be installed onto a USB device; installing it onto your hard disk is perfectly fine.
Scroll down the page linked to above and look for the Legacy Versions section to download older versions.
I also use a Portable Firefox as a seperate 'dev' version which is loaded with all of my dev-centric add-ons. This keeps my normal day-to-day Firefox clutter-free and much lighter.
(I haven't tried this approach with 3.5, but this worked with 2.0 and 3.0.)
Just install any desired versions of Firefox to their own directory. As long as you use a different profile for each version, there shouldn't be any issues with the various versions co-existing.
Once you have created the profiles for each version, you can specify the profile Firefox should use so that you don't have to chose one every time. See MozillaZine for the list of Firefox arguments.