Thank you for your post.
From the information you provided, it is difficult to determine where the problem exists.
If you set up the mail client as Outlook, do you still encounter the problem?
If you create a new database file and select Send Mail from the File menu, do you still encounter the error?
Are any other applications running besides FileMaker Pro 9 Advanced and Thunderbird?
Have you tried uninstalling FileMaker Pro 9 Advanced and reinstalling?
I'm trying to narrow down the possible causes by looking at Thunderbird, the database file, other applications conflict, and fresh install of FileMaker, respectively.