Do you get a message that the profile folder cannot be found or is not accessible? This can happen if you remove the profile folder, but leave the profiles.ini file that registers profiles. Locate the profiles.ini file and delete this file or use the Profile Manager to create a new profile. See: *https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible <hr> You can delete the profiles.ini file to make Firefox create a new (default) profile or use the Profile Manager to create a new profile or register an orphaned profile. You can recover an orphaned profile via these steps: *use the Profile Manager to create a new profile *click "Choose Folder" in the Profile Manager *browse to the location of a lost profile to recover this profile and register it in the Profile Manager Note that you should never use "Choose Folder" to specify a folder that already contains files that do not belong to Firefox (i.e. only use "Choose Folder" to recover an orphaned profile that isn't listed in the Profile Manager) You can find the profiles.ini file in this hidden location: *Windows: %AppData%\Mozilla\Firefox\<br>C:\Users\&lt;user&gt;\AppData\Roaming\Mozilla\Firefox\ *Mac: ~/Library/Application Support/Firefox/ *Linux: ~/.mozilla/firefox/ If deleting the profiles.ini file isn't working then also check the location of the cache files. *Windows: C:\Users\&lt;user&gt;\AppData\Local\Mozilla\Firefox\Profiles\&lt;profile&gt;\ *Mac: ~/Library/Caches/Firefox/Profiles/ *Linux:~/.cache/mozilla/firefox/ You can also try to rename/delete the Mozilla\Firefox\ folder in both above mentioned locations (main, cache). <hr> *https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles *http://kb.mozillazine.org/Profile_folder_-_Firefox