The default should be to use the OS' DNS resolution library. DoH is almost like a VPN; AFAIK Firefox's proxy settings default to using the proxy server of the OS, so why should DNS be any different? I don't care if it's encrypted or not, I care whether applications respect the user by using the system's configuration. If I have the system configured to use a VPN, I expect all applications' traffic to get tunneled through it. The same for DNS and TCP and a lot of other services that the OS provides.