It seems to me that upstream made it clear, back in September 2013, what they want to see done before they can merge these patches. I've seen no answer from "our side" since then, so it's unclear to me what's the current status, and whether someone is actively working on it.
Note that this determines, in part, the Tails roadmap to ship Thunderbird, so it would be very useful for us to have a clearer view of who's committed to do what in this area, and when.