You can check for problems with the profiles.ini file that registers profiles and the profile marked as Default=1 is no longer present. You can try these steps: *delete the profiles.ini file to make Firefox create a new (default) profile *use the Profile Manager to create a new profile<br>https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles *register a profile that isn't listed in the Profile Manager You can find the profiles.ini file in a hidden location: *Windows: %AppData%\Mozilla\Firefox\<br>C:\Users\&lt;user&gt;\AppData\Roaming\Mozilla\Firefox\ See: *https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible You can possibly recover personal data from a profile in the "Old Firefox Data" folder on the desktop created by using the Firefox refresh feature or from from another profile. *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile