those of us who use VPN clients
well, it means DNS selection is built into the browser,
so whatever DNS your VPN has you use won't matter.
the fact that DNS is encrypted again by the VPN tunnel
doesn't matter - all your httpS is pre-encrypted as well.
the bad news here is if you use your VPN to occasional
get away from your normally security restrictive DNS,
then you won't want to use Firefox.
bottom line is, Firefox will be good for people naive
about their online security. the rest of us may want
to deal with it further up the pipe on our own.
what worries me is that browser built in DNS hosts
will eventually be corrupted by ad hosts who pay them
to let their traffic through. it's how some ad blockers
are already exploiting the trust from their app users.
thus another reason you want to control your own DNS
not "trust" your browser to do it.