How to install Firefox 3.5 along with 3

The best way to do this is install Firefox 3.0 on your computer, then download Portable Firefox to run version 3.5.

"(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."