This happens if the profiles.ini is missing. *http://kb.mozillazine.org/profiles.ini_file You can check the permissions of the folder that stores the file to make sure that you have write access. * /Users/<user>/Library/Application Support/Firefox/ *http://kb.mozillazine.org/Profile_folder_-_Firefox Try to repair the disk permissions: *http://thexlab.com/faqs/repairprocess.html ---- In Mac OS X v10.7 and later, the $HOME/Library folder is a hidden folder. Open Finder and use one of these: * Go > Go To Folder (Shift-Command-G) and in the dialog type: ~/Library * Open the "Go" menu and hold down the Option key to make the Library appear