Thank you for your post.
At this time, FileMaker Pro uses MAPI compliant mail programs. Thunderbird is not MAPI compliant, so therefore, it will not work.
Then at least I know.
It turns out that it is possible to solve this.
If you change the default mail application in both the Internet Control Panel as well as the Add/Remove Software Control Panel, it works!
There are two types of MAPI implementations, SimpleMAPI and ExtendedMAPI. ExtendedMAPI uses MAPI as a actual messaging engine. It used by Outlook to make native calls to the Microsoft Exchange server. Outlook Express and Thunderbird don't support Extended MAPI, which is why you can only use them with a Microsoft Exchange Server if the admin also installs POP/IMAP/SMTP mail servers on that server.
SimpleMAPI is a subset thats used by applications to call the default email client to send a message without that application having to know anything about what email client is doing the actual work. For example, if Thunderbird was the default email client you could use Microsoft Word, Microsoft Excel, or MYOB to send a message. Many scanners also use SimpleMAPI to mail an image file.
All you need to do to support SimpleMAPI is to configure Thunderbird as the default email client and make certain you don't configure any other email clients that you might run as the default email client. Otherwise the email clients will fight to be the default email client.
That worked! Thanks for being the one to provide a solution carlsson (and google!)