This will first happen for users in the United States in the Fall of 2019. If a user has chosen to manually enable DoH, the signal from the network will be ignored and the user’s preference will be honored. (quoting from the link @AKTheKnight shared in the first post of this discussion).
the signal from the network and Fall of 2019 makes me a bit nervous.
So Mozilla is able to remote control some(/several/all?) Firefox core settings with some broadcast signals? Or does this only apply to those who have enabled Studies ? If so, then it would probably reach only a minority of people as I seem to recall that studies are an opt-in feature.
However, we had this major Firefox issue some while ago where addons didn't work any more. Mozilla's solution was to tell people to enable studies so they can import a fix. I believe many followed guides on the web how to enable studies to get their issue fixed and never looked at it again. It wouldn't surprise me if this move would have increased the studies coverage from like 0.1% to 25% (these numbers are purely fictional).