do not ask to be the default client
(Another "prevent them from shooting their own feet"...)
Thunderbird asked whether I want it to be my default client for mail, news, whatever. No. And the question is quite pointless.
Assumed people use their non-anonymous stuff and their anonymous stuff on the same system, it might happen they (accidentally) click a e-mail link with the non-anonymous browser. That should not fire up Thunderbird / TorBirdy. If they accidentally hammer the enter key or click on that button while Thunderbird just showed the question, they see nothing but a short flash and activated it.
Please disable the question and being the default client for anything by TorBirdy-defaults.
This is NOT a request to forbid explicitly setting to be the default client if the user really wishes.