If none of the below helps, you can resume where I started, and left off....
Stolen Acquired from here:
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.
To quote "Westside" from here:
Did you try thunderbird.exe in Processes. You will not see anything in
the Application tab, but only under Processes. That happens,
frequently, with my Firefox, and removing the .exe, will do the job. I
have no idea about restarting the computer. I would expect that it
would close the process, but I never tried that approach.
Another option, that I just made-up:
Just move that profile you imported and see about importing it from your Thunderbird while it's running..