Firefox accepts a bootstrap IP, or uses the system resolver:
> network.trr.bootstrapAddress
> (default: none) by setting this field to the IP address of the host name used in "network.trr.uri", you can bypass using the system native resolver for it. Use this to get the IPs of the cloudflare server: https://dns.google/query?name=mozilla.cloudflare-dns.com
> Starting with Firefox 74 setting the bootstrap address is no longer required in mode 3. Firefox will attempt to use regular DNS in order to get the IP address of the trusted resolver. However, if DNS resolution of the resolver domain fails, setting the bootstrap address is again necessary.
How is the IP address of the DoH server obtained?