I hate CloudFlare as much as the next guy but you can change the DoH server, can't you? I hope Firefox will expose the option in the interface and not in about:config. But we all know how Mozilla are...
For Mozilla, I'm not sure, but they often follow Google's lead, and there is a strong case for Google to go that route
In any case if it ever starts defaulting to on in any browser, it's very likely the others will follow suit, as it's easy to imagine quite a lot of PR around the security benefits of the brave new world