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

Locate the profiles.ini file and delete this file or use the Profile Manager to create a new profile.

See:

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

This issue can be caused by a problem with the profiles.ini file or with the permissions (owner) of involved folders with the main profile location or the location used for the cache.

You can use one of these to make Firefox create a new default profile or recover an existing profile:

• Delete the profiles.ini file to make Firefox create a new (default) profile
• Use the Profile Manager to create a new profile
  If you still have an existing profile then click "Choose Folder" in the Profile Manager and browse to the location of a lost profile to recover this profile
  https://support.mozilla.org/kb/Managing+profiles

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

• Windows: %AppData%\Mozilla\Firefox\
  *C:\Users\<user>\AppData\Roaming\Mozilla\Firefox\
• Mac: ~/Library/Application Support/Firefox/
• Linux: ~/.mozilla/firefox/

If clearing the profiles.ini file isn't working then also check the location of the cache files.

• Windows: *C:\Users\<user>\AppData\Local\Mozilla\Firefox\Profiles\<profile>\
• Mac: ~/Library/Caches/Firefox/Profiles/
• Linux:~/.cache/mozilla/firefox/

You can also try to rename/delete the MozillaFirefox folder in both above mentioned locations (main, cache).

• http://kb.mozillazine.org/Profile_Manager
• https://support.mozilla.org/kb/Managing+profiles
• http://kb.mozillazine.org/Profile_folder_-_Firefox

Thanks very much, but from my initial post you can see that I have in fact created new profiles with the profile manager. The problem is, that (for me) the profile manager will run only with root privileges, and any profile it creates then seems to be completely inaccessible to Firefox run as a normal user.

This may be a bigger problem I'm having with permissions in El Capitan, a few other applications that should run fine are also unable to access the filesystem normally. If nobody else has seen this behavior it may be time to re-install OS X.

Try to repair the disk permissions:

• http://thexlab.com/faqs/repairprocess.html

Thanks Cor-El. As of OS X El Capitan, Apple no longer permits/requires repairing permissions. Disk Utility now has no such option. Apparently they think that the critical permissions are now invulnerable. (cf., SS Titanic).