I don't think Mozilla will end up forcing a single DNS endpoint, likely, they will have a GUI to configure this. If I had to guess, they will likely have a heu

I don't think Mozilla will end up forcing a single DNS endpoint, likely, they will have a GUI to configure this. If I had to guess, they will likely have a heuristic where your local DNS is first checked somewhat regularly if it supports DoH, if not use the one configured in Firefox, if DoH is enabled that is.