Create a new profile exclusively for the Firefox 4 version and create a desktop shortcut with -P "profile" appended to the target to launch that profile.

• http://kb.mozillazine.org/Testing_pre-release_versions

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox

Thanks. The problem was FF4 was loading an incompatible theme. I'm not at all impressed with this new version. I see no perceptible increase in speed, most of my add-ons work only because I force them to, and a few won't work at all. Not to mention that as I just discovered, my favorite theme Chromifox Extreme, breaks FF4 completely. I have gone back to 3.6