I suspect the plan is to make ad blockers impotent by reducing their ability to adapt quickly to new forms of block evasion

the ad blockers slowly become more and more useless, people stop using them entirely, and google's the biggest winner

then I'll use a fork that uses old DNS. maintaining a small diff that enables old DNS is easy enough that I could do it myself. much easier than trying to fork out Manifest V3.

Firefox has no incentive to prevent people from blocking ads, and DNS over HTTPS isn't a tool to block users from reconfiguring their own machines. All browsers let you configure your DNS settings still. The thing that DNS over HTTPS prevents is letting whoever is supplying your internet (including your ISP, whatever hotspot you're using on the go, etc) manipulate your DNS. It's not about removing control from the person who owns the machine.