I was using latest version of Firefox 3.6.3, but wanted to install a add on which works only on Firefox 3.5. But I didn’t want to uninstall my latest Firefox . Solution for this was to install Firefox 3.5 into a different folder from the existing Firefox installation. Then I started the Firefox 3.5 with -P option, to get the Profile dialog and created a new profile for Firefox 3.5. Then started Firefox 3.5 with the new profile.
Running different versions of Firefox