Easiest is to install a portable Firefox 3.0.x version. The portable version uses its own profile and that way you do not have to make sure that the 3.0 version doesn't use the Firefox 3.6 profile.

You can do a custom install of the Firefox 3.0.19 version, but then you have to make sure that you do not start the Firefox 3.0 program after installing and first create a new profile for that version and create desktop shortcuts to start each version with its own profile to avoid corrupting files in the profile folder.

See:

• http://kb.mozillazine.org/Profile_Manager
• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile

You can get Firefox 3.0.19 version of Firefox Portable here. Scroll down to Legacy Version 3.0:
http://portableapps.com/apps/internet/firefox_portable/localization#legacy

The Firefox Portable support forum is here:
http://portableapps.com/forums/support/firefox_portable

That's a great solution, I wanted to avoid multiple profiles and still have both versions at my disposal....and it's people like you that help keep me away from using explorer! Thanks so much.