TBB's Firefox should use optimistic data socks handshake variant

Tor Proposal 181 lets the Tor client save a round-trip if the application speaks socks in a special way. In short, the application needs to send its data before hearing that the socks connection was successful. It's supported as of Tor 0.2.3.3-alpha.

Ian originally had suggested to hack polipo to use a modified socks handshake. With polipo out of the picture for TBB, we should make Firefox itself do it.

Is this something Torbutton should (could) do, or should we patch the Firefox we include in TBB?

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