Any other ideas are very welcome...
Open your ~/.mozilla-thunderbird/...default/prefs.js and add
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");
/usr/bin/firefox might be replaced if firefox resides somewhere else.
Cheers,
Florian