ThunderFix is a small FREE tool which cleans the junk in thunderbird and improves the thunderbird performance to a great extent. This tool could be of great help specially if you are facing slow start time in thunderbird.
ThunderFix detects all the .msf files with in your thunderbird profile and removes them, so the next time when you run thunderbird it will generate the new MSF files and you will see the improvement in the thunderbird startup time.