Solution to infamous "Firefox is already running but is not responding." problem! Ubuntu 14.04

Try this simple solution if you have received this error:

Firefox is already running but is not responding. You must either close Firefox or restart your system.

 
This worked on Ubuntu 14.04.

First:

Run:
$ ps aux | grep firefox 

and kill the returned processes with PID:
$ kill PID

If the problem still persists:

$ cd ~
$ cd ./mozilla
$ cd firefox
$ ls

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.