We should be watching system add-on uptake in a way consistent with how we track other sorts of updates. Telemetry and redash (sql.t.m.o) seem like the best tools for this right now.

We have a pretty good idea of the uptake of system add-ons now: https://sql.telemetry.mozilla.org/queries/2654#5575 There is a persistent ~6% of users that don't seem to get the update. These users have Telemetry enabled and also update (incl. auto-update) enabled. The next step is to ship more diagnostic code so we can figure out why this is the case (bug 1307568).