If you have used System Restore then Windows may have restored the profiles.ini file to a previous version that doesn't know about the new profile that the reset created. You can check which profile folders are currently present in the location that Firefox uses and see if you can re-register that profile by creating a new profile and click "Choose folder" to select the location on that lost profile. *http://kb.mozillazine.org/Profile_folder_-_Firefox *http://kb.mozillazine.org/Profile_Manager *http://kb.mozillazine.org/Recovering_a_missing_profile In case you need to move over data from a previously used profile, see: *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox