Not sure whether it works the same with version 2, but check out the coexistence advice for versions 3 and 4: can I install versions 3 and 4 on my PC at the same time? | Firefox Support Forum | Firefox Help.

And why are you still running Firefox 2?

Firefox 2.0.0.20 is way behind on security updates as it was released way back in December 18, 2008.

You can have multiple builds of Firefox releases by installing them in separate folders (custom install) and use separate Profiles for each if you wanted. I strongly suggest you use separate Profiles for 2.0.0.* and 4.0.1 as otherwise the Profile that was used will be well used regardless of Firefox version unless selected otherwise and Profile corruption can occur if going back and forth two very different Major versions with same Profile.

You will need to either start Firefox with the ProfileManager and select the appropriate Profile to start with or to have it start with a specific Profile.

http://kb.mozillazine.org/Profile_manager

http://kb.mozillazine.org/Profile_manager#Shortcut_to_a_specific_profile