You need to locate the profiles.ini file or use the Profile Manager to create a new profile. See: *https://support.mozilla.org/kb/how-run-firefox-when-your-profile-missing-or-inacc ---- This can be 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. Do you see any profiles in the location that is used for profiles? *C:\Users\&lt;user&gt;\AppData\Roaming\Mozilla\Firefox\Profiles\&lt;profile&gt;\ Note that "AppData" is a hidden folder. *http://kb.mozillazine.org/Show_hidden_files_and_folders 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 />If you still have an existing profile then click "Choose Folder" in the PM and select the location of a lost profile and recover this profile. You should find the profiles.ini file in this hidden location: *Windows: %AppData%\Mozilla\Firefox\<br>C:\Users\&lt;user&gt;\AppData\Roaming\Mozilla\Firefox\ *Linux: ~/.mozilla/firefox/ *Mac: ~/Library/Application Support/Firefox/ ---- *http://kb.mozillazine.org/Profile_Manager *https://support.mozilla.org/kb/Managing+profiles *http://kb.mozillazine.org/Profile_folder_-_Firefox