I just noticed that Firefox has expanded (and relocated in the Settings) the DoH (DNS over HTTPS) settings and added some new options. If you haven’t checked/updated yours, not time like the present. Go into Settings, select the “Privacy & Security” tab and scroll to the bottom.
I have DoH blocked at the firewall, otherwise it would be useful.
I use a Pihole with lots of blacklisted sites and it in turn uses the same Quad9 servers, but over DNS over TLS with DNSSEC.
How are you identifying DoH to block it?
I block https to the known DoH servers.