Cant start Firefox, profile missing or inaccessible error msg

If you are having a problem with Firefox then sometimes, rather than trying to find and fix the exact cause of the problem, it is easier just to make a new Firefox profile

But before creating a new profile you should be sure that your existing Firefox user profile data, which includes bookmarks, passwords, cookies, preference settings and added extensions, will no longer appear in the new profile.

To create a new default profile:

Press Windows+r to open the run dialog box, type in: firefox.exe -p This will bring up a new window, create a new default user and the profile error will be gone.