When it's run Firefox places a file in your profile directory to indicate the profile is in use. The message "Firefox is already running, but is not responding..." indicates that your Firefox thinks your profile is already in use. There are several possible causes of this.

• You have Firefox running somewhere already. You can't run Firefox on more than one computer at the same time with the same profile. If you are logged in to another computer and are running Firefox there then this is the cause of the message. Quit Firefox on the other computer.
• If you have had Firefox running since you logged in and quit it or it crashed, it may be that it did not exit properly and is still actually running even though you can't see it. At a command prompt run the commands

  $ killall firefox-bin
  
  $ killall firefox

  Then try running Firefox again.