See http://kb.mozillazine.org/Browser_will_not_start_up ---- Are you using a fresh, separate, profile for the Firefox 4 beta version or do you share the profile with the Firefox 3.6.x version? If you share the profile then 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