It's gonna be everywhere. Chrome is rolling it out too.
Chrome is only enabling it as an upgrade, not as a replacement. I.e. if the DNS server you use supports DoH, then it will upgrade to DoH. But if you use OpenDNS (which does not support DoH) for blocking, then it won't touch your settings, and will stick with it.
This is why Firefox's implement is so retarded, compared to everyone else, as they will blindly override any filtering DNS server you might have implemented on your network. It's screwing up with network management, by blindly overriding it, unless your DNS provider happens to support their canary system.