Hello sbreymaier, try to manually delete the profiles.ini file from firefox profile folder path to force firefox to create a new profile, so to be able to run firefox again.

see in the next link the Method 2: Manually delete the profiles.ini file for more information : https://support.mozilla.org/kb/how-run-firefox-when-profile-missing-inaccessible#firefox:win8:fx39

thank you

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

• Delete the profiles.ini file to force Firefox to 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 PM and select the location of a lost profile and recover this profile.

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\
• 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