A podle toho návodu to nefunguje?

Put these lines in user.js file, in your Thunderbird profile:

user_pref("network.protocol-handler.app.http", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.https", "/usr/bin/firefox");
user_pref("network.protocol-handler.app.ftp", "/usr/bin/firefox");