Måske noget med disse linier i all-thunderbird.js?
// Expose only select protocol handlers. All others should go
// through the external protocol handler route.
pref("network.protocol-handler.expose-all", false);
pref("network.protocol-handler.expose.mailto", true);
pref("network.protocol-handler.expose.news", true);
pref("network.protocol-handler.expose.snews", true);
pref("network.protocol-handler.expose.nntp", true);
pref("network.protocol-handler.expose.imap", true);
pref("network.protocol-handler.expose.addbook", true);
pref("network.protocol-handler.expose.pop", true);
pref("network.protocol-handler.expose.mailbox", true);