Hi,
I'm part of the people working on Tails and I have a doubt about Tor Browser that I couldn't solve by reading your design document.
Are automatic updates performed on additional extensions that the user might install in Tor Browser? The design document only mentions that "addon updates" are proxied as well which might mean that they still happen but it's not explicit. According to Giorgio Maone, a regular Firefox checks for updates of the extensions once per day but we were not sure whether Tor Browser prevents this for some reason.