remove dependency on http to socks forwarder
For what is the http to socks forwarder needed anyway? (privoxy)
If I don't assume wrong, I think Firefox and Thunderbird's share a lot network related code. Firefox+TorButton do not require a http to socks forwarder. Why does Thunderbird+TorBirdy need one?
You could reduce some complexity by sending everything over socks.
Perhaps this would also help solving #6359: make use of stream isolation.