Starting Mozilla, MozillaFirebird or another Mozilla based browser suddenly does not work anymore. Instead of the browser window, a dialog pops up saying Select User Profile. But if you do this, you are getting told: Mozilla cannot use the profile “default” because it is in use. Please choose another profile or create a new one.
You had killed Mozilla the hard way, e.g. with kill -9 or by hard-switching of you Linux box. Mozilla keeps a lock file in order to make sure each profile is in use only once at the same time. This lock file is normally deleted automatically but has been left over due to the hard switch-off.
The lock file is located in your home directory either under .mozilla or under .phoenix (if you are using MozillaFirebird):
> ls .phoenix/*/*/lock
Klick away the dialog, klick Exit for now. Then delete the lock file (which is a symbolic link):
> rm ~/.phoenix/*/*/lock
Now you should be able to start Mozilla again.