They can just configure their internal DNS server to return NXDOMAIN for use-application-dns.net, which is a canary domain that Firefox checks before using DoH:

They can just configure their internal DNS server to return NXDOMAIN for use-application-dns.net, which is a canary domain that Firefox checks before using DoH: https://support.mozilla.org/en-US/kb/canary-domain-use-application-dnsnet https://support.mozilla.org/en-US/kb/canary-domain-use-appli...