Their concern is (AIUI) very much that browsers are much more of a "general" product, and so used by a greater proportion of users (as small as FF's market share as a %age might be, that's still a lot of people).
Also, it's not ultimately just going to be Firefox that supports this.
Though, if we take that first argument, Opera is a browser and I don't seem to remember them complaining when it got VPN functionality built into it.
The problem with ISPs offering their own DoH server is that (as it stands) you'd need to manually configure it, there's no way for them to push an auto-config down to you. They'd also need to be globally accessible/usable so that when you go out and about onto other networks with your mobile/laptop resolution doesn't break (because that new network can't autoconfig you either).
It's not something that can't be addressed, but does still complicate things a bit
Biting the hand that feeds IT
