[ocdtrekkie]

I believe Pihole automatically put in the NXDOMAIN entry needed to disable DoH on Firefox for you. But who knows how long Firefox will respect that, since there's nothing stopping ISPs from employing the same strategy to disable DoH.

[josephcsible]

Even if Firefox stops respecting that due to malicious ISPs using it, you'll always be able to turn off DoH in Firefox's preferences.