I found this thread without a clear answer. I did however find an answer some place else and thought I would share it here.
$ cd ~/.mozilla/firefox/*.default
$ vi user.js
Add line:
user_pref("network.protocol-handler.app.mailto","/usr/bin/thunderbird");
Stop and start firefox and tbird will be your default email client.
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This is an email sent via the webforum on http://fcp.homelinux.org
http://fcp.homelinux.org/modules/newbb/viewtopic.php?post_id=104523&topic_id=24655&forum=10#forumpost104523
