What is your operating system? Windows?

It sounds like the profile cannot be created or accessed by Firefox. If this is on Windows, it might be due to not running Firefox or the installer with sufficient privileges; the fix is to try running either as an administrator with the Run as administrator option: http://www.sevenforums.com/tutorials/11841-run-administrator.html.

You can also try starting Firefox in Safe Mode by holding Shift before you start Firefox. Check Task Manager (again, if on Windows) to see that no instances of firefox.exe are running.

This article has suggestions for what to do if your profile cannot be found: https://support.mozilla.org/en-US/kb/how-run-firefox-when-profile-missing-inaccessible.

You need to 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 is usually caused by a problem with the profiles.ini file.

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/

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