Here’s a quick how-to on getting Firefox to start new mail messages with Thunderbird and getting Thunderbird to open links in Firefox under Linux.
For Thunderbird:
All you need to do is edit the prefs.js file in your .mozilla-thunderbird/blahblah.defaults/ (or similarly named) directory. Add the following (specifying your path to Firefox):
user_pref("network.protocol-handler.app.http", "/usr/bin/firefox" );
user_pref("network.protocol-handler.app.https", "/usr/bin/firefox" );
For Firefox:
Edit the prefs.js file in your .mozilla/firefox/blahblah.defaults/ directory. Add the following:
user_pref("network.protocol-handler.app.mailto","/usr/bin/thunderbird");