Have you tried deleting the .parentlock file?

http://kb.mozillazine.org/Profile_in_use#Remove_the_profile_lock_file

That has happened to me. Tried the parentlock solution and tried ending the task. Only a reboot would solve it.

Can't delete parent.lock because Thunderbird is still running.

I live in an area where there are occasional power cuts. Every time I get a power cut Thunderbird tells me it can't open because it is already running. I do the whole %AppData% thing and try to delete parentlock, and get a message saying I can't delete parentlock because...Thunderbird is still running. So I have to uninstall Thunderbird and reinstall it. I have had to do this four times already and nobody seems to have a solution.

I can't believe I am alone in having this problem. Can't the guys at Mozilla fix this? None of my other programs/apps have this problem.

Help, please,

Conor