Add this to ~/.thunderbird/8ee0ulse.default/prefs.js <---yours may be different
Code:
user_pref("network.protocol-handler.app.ftp", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.http", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.https", "/usr/bin/firefox");
Thats for firefox if you have /usr/bin/firefox may be /usr/bin/mozilla-firefox
Make sure thunderbird is closed when you edit the file ...