Why don't they rather include a resolver in Firefox ? This way, no privacy problems, you're directly contacting authoritarive servers. And you don't rely on a single dns-over-https provider. Is the latency a big problem there? I would say that with caching it is not too bad.
2. There's plenty of networks that don't let arbitrary DNS traffic out, so this doesn't work without another fallbacks. Fallbacks for security features are bad.
I see that there are legit controversies around DoH. But these "Why don't you just do X?" comments aren't helpful. Try to understand the problem they're trying to resolve.