See http://kb.mozillazine.org/Locked_or_damaged_places.sqlite Create a new profile exclusively for the 4.0 beta 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