I wanted to disable Firefox' DNS-over-HTTPS in my network so that my custom home domains still work.
It is possible to do that network-wide by preventing the canary domain use-application-dns.net from getting resolved (NXDOMAIN).
I use Dnsmasq on my home server, and to disable the canary domain I only had to add the following line to the configuration file:
/etc/dnsmasq.conf
address=/use-application-dns.net/