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

• You have Thunderbird running somewhere already. You can't run Thunderbird on more than one computer at the same time with the same profile. If you are logged in to another computer and are running Thunderbird there then this is the cause of the message. Quit Thunderbird on the other computer.
• If you have had Thunderbird 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 thunderbird-bin
  
  $ killall thunderbird

  Then try running Thunderbird again.
• You don't have Thunderbird running somewhere else and your profile is marked as being in use even though it isn't. Run Remove Thunderbird Profile Lock from the Application Browser. This will remove the lock file from your profile.