Today, I went around and around with Firefox.  It game me the message:

"Firefox is already running, but is not responding. To open a new window, you must first close the existing Firefox process, or restart your system. "

The Firefox process was not running.  So, I tried rebooting and presto, same error.  I uninstalled and reinstalled Firefox and the same message continued.

After a bit of searching, the issue I ran into had something to do with a messed up profile.  I used the RUN command in Windows and entered:

Filefox.exe -p

This allowed me to create a new profile and things were good again.

You might also take a look at:

http://kb.mozillazine.org/Migrating_settings_to_a_new_profile