if you don't mind losing your data (password, bookmark, history), you can goto firefox profile folder and delete everything. https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data?redirectlocale=en-US&redirectslug=Profiles

See: *https://support.mozilla.org/kb/how-run-firefox-when-your-profile-missing-or-inacc This is usually caused by a problem with the profiles.ini file and the profile marked as Default=1 in this file is no longer present on the hard drive. *Windows: %AppData%\Mozilla\Firefox\<br>C:\Users\&lt;user&gt;\AppData\Roaming\Mozilla\Firefox\Profiles\&lt;profile&gt;\ *Linux: ~/.mozilla/firefox/ *Mac: ~/Library/Application Support/Firefox/ You can use one of these to make Firefox create a new default profile or reuse an existing profile: *Delete the profiles.ini file to force Firefox to create a new default profile *Use the Profile Manager to create a new profile<br />Use "Choose Folder" when you create a new profile to select the location of a lost profile and recover this profile *http://kb.mozillazine.org/Profile_Manager *https://support.mozilla.org/kb/Managing+profiles *http://kb.mozillazine.org/Profile_folder_-_Firefox