I recently reinstalled Ubuntu Mate, and copied over my Thunderbird profile from my USB stick back to the .thunderbird directory under my home folder.
I then started Thunderbird and received the following error.
I first checked to see if Thunderbird was still running in the background by running top from the terminal, but couldn't see any Thunderbird processes running. I also ran the following command just to make sure.
killall thunderbird
I also restarted the machine, but the error still occurred when I tried to launch Thunderbird.
I then decided to check the permissions of the folders, and noticed they where owned by root.
I then opened my terminal and ran the following command and changed the permissions of the folders and files to my username.