Firefox is already running but is not responding. You must either close Firefox or restart your system.
This worked on Ubuntu 14.04.
$ ps aux | grep firefox
and kill the returned processes with PID:
$ kill PID
If the problem still persists:
$ cd ~
$ cd ./mozilla
$ cd firefox
now you will see a directory with extension ".default". The filename is a combination of letters and numbers. CD into that directory.
$ cd [filename].default
$ ls -a
now you will see a file named lock (or .lock). delete that. Problem is solved.
$ rm .lock
Problem should be solved by now.