I mostly don't like this
I love the idea of DNS over HTTPS - I have it enabled myself in PiHole, using applied-privacy.net as my DNS over HTTPS provider, it works great. My devies uses PiHole as their primary DNS server, so no need to fiddle with DNS on an application level. DNS at application level seems weird,
I don't like that Mozilla are changing something so central by default; even if it's done with good intentions, it normalises such things and then before you know it, less ethical players such as Google will follow suit.
I don't like that Mozilla are changing the behaviour of their software deliberately based on your territory, that feels a bit weird.