There might be a problem with permissions/ownership of involved folders/files preventing access. You can check for problems with the profiles.ini file that registers profiles. See: *https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible <hr> 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 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/ You can recover an orphaned profile via these steps: *use the Profile Manager to create a new profile<br>https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles *click "Choose Folder" in the Profile Manager *browse to the location of this profile to register this profile in the Profile Manager and use this profile See "Profile was moved or renamed": *https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible Note that you should never use "Choose Folder" to specify a folder that already contains files that do not belong to Firefox. Only use "Choose Folder" to recover an profile that isn't listed in the Profile Manager. If this didn't work then check the read/write permissions and owner or remove the Mozilla/Firefox folder in both main/root and cache locations. 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 check if you have an "Old Firefox Data" folder on the desktop in case the Refresh feature in Firefox has been used and a new profile has been created. *https://support.mozilla.org/en-US/kb/reset-firefox-easily-fix-most-problems *https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile <hr> *http://kb.mozillazine.org/profiles.ini_file *https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles *http://kb.mozillazine.org/Profile_folder_-_Firefox