Turn on the background autoupdater
Now that we're fairly sure that everything is working OK with our offline mar signing, etc, we should turn on the background updater for 5.0. The current popup for updates is still a barrier for some people to update immediately, I suspect.
I will test an upgrade from 5.0a2 to 5.0a3 with app.update.auto set to true on Linux, so we can verify it still works across the FF31 to FF38 transition. If that works out, we should set the pref by default in 5.0a4, and again in 5.0.
I might need someone else to test Windows and Mac.