Programs in the Mozilla suite control running multiple copies of the same program by using lock files. It looks like Thunderbird and/or Firefox crashed and left a lock file(s) set. Then when you try to start them again the new copy finds a lock set and thinks that there is a copy already running.
To check this out, have no copy of any Mozilla based program running. Use this command to find any errant lock files:
find /home/username/.mozilla -iname "*lock*"
Delete any lock files that you find. Then try starting the program again.