Right now, --backgroundtask backgroundupdate will happily stage an update and exit. The next time it (or Firefox) runs, the staged update will be applied, completing the update. I'm not aware of a technical reason to do this in two steps. We elected to do so out of an abundance of caution: what happens if we get stuck in some kind of restart loop, constantly invoking the background task?

This ticket tracks doing better. We can try to restart the task after staging, or we can turn off staging in the background task and always apply, or maybe something else.

This doesn't block our initial release (i.e., milestone 1) -- it's just an improvement.

I had this spontaneous idea, that maybe the task scheduler could also be used to schedule a preparation task to apply the update before the user starts Firefox again. I believe the scheduler can schedule tasks for the next idle state of the machine. There may be reasons against it and I already acknowledge, that it could make the testing more difficult, but it would probably reduce the felt startup times for some users.

https://hg.mozilla.org/mozilla-central/rev/d82a8644ef7e