When Firefox downloads an update, it previously kept the update around to apply
it on the next restart. This patch changes this so that the updater program
is launched in the background as soon as the update has finished downloading
in order to stage the updated version of the application by copying the
existing installation directory to a temporary location and applying the update
on top of it, and replace the existing installation directory with the staged
directory on the next restart.
Because the replacing step is typically very fast, this patch eliminates the
wait for the update to be applied on restart, making it unnecessary to show a
progress dialog when restarting.
