http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

https://support.mozilla.org/en-US/kb/profile-manager-create-and-remove-firefox-profiles

https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

https://support.mozilla.org/en-US/kb/back-and-restore-information-firefox-profiles

Thanks, but I already referenced two of those links in my question and the other two aren't helpful.

• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data
• https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles
• http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues

If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over the problem. Never work with original files.

• https://support.mozilla.org/en-US/kb/recovering-important-data-from-an-old-profile
• http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox

Please let us know if this solved your issue or if need further assistance.

You need to check the permissions and owner of the profiles.ini file and the Profiles folder.

You can find the profiles.ini file in this hidden location:

• Mac: ~/Library/Application Support/Firefox/

Firefox uses two locations for the Firefox profile folder, so make sure to look in the correct location. Location used for the main profile that keeps your personal data (Root Directory on about:profiles).

• ~/Library/Application Support/Firefox/Profiles/<profile>/

Location used for the disk cache and other temporary files in "AppData\Local" (Local Directory on about:profiles).

• ~/Library/Caches/Firefox/Profiles/

I deleted my profiles.ini file based on the advice in this article: https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible

I don't need to copy any previous data over, I just want to get a fresh profile running.

The only folders I have in ~/Library/Application Support/Firefox/ are 'Crash Reports' and 'Pending Pings'. The only way I can get Firefox running is to use sudo /Applications/Firefox.app/Contents/MacOS/firefox-bin -P - but this does not create a profiles.ini file anywhere and Firefox wont start without the sudo terminal command.

Download the full version and trash the currently installed Firefox application to do a clean reinstall.

Download a new copy of the Firefox application and save the Disk Image file to the desktop

• Firefox 57.0.4: *https://www.mozilla.org/en-US/firefox/all/

• trash the currently installed Firefox application (open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean reinstall
• install the new version that you have downloaded
• https://support.mozilla.org/en-US/kb/Installing+Firefox+on+Mac

Your personal data like bookmarks is stored in the Firefox profile folder, so you won't lose personal data When you uninstall and reinstall or update Firefox, but make sure NOT to remove personal data When you uninstall Firefox as that will remove all Firefox profile folders and you lose your personal data.

I've tried that unsuccessfully. I went further to delete the Firefox folder in ~/Library/Application Support/Firefox/ as well. When I do a clean install Firefox and try to launch it:

1. A folder is created in the Application Support directory, but, as before, it only contains 'Crash Reports' and 'Pending Pings' - no profiles.ini is created.
2. I get the same pop up message "Profile Missing: Your Firefox profile cannot be loaded. It may be missing or inaccessible."

Then Firefox quits.

This may be a symptom of some other problem, as this morning Evernote has thrown an error when trying to update, being unable to write to the caches folder. I'll update this question if I manage to fix it.

In case anyone else runs into this problem, I seem to have had a problem with user permissions in my account, since I could run firefox in a new admin account without any trouble. I fixed the user permissions by following this advice.

I'm having the same issue. @soundproof: what OS are you running, and what part of the stuff on that page worked for you?