All preferences for the DNS-over-HTTPS (DoH) functionality in Firefox are located under the "network.trr" prefix (TRR == Trusted Recursive Resolver). The support for these landed in Firefox 60 and has been improved in Firefox 61. Goto about:config to set these preferences.
Some users have reported needing to restart to have the changes take effect. The DNS tab of the about:networking page will indicate whether a name was resolved via TRR or not.
set this preference to 2 to enable DoH in a mode where DoH is used as the first choice for DNS, but it will still fallback to native DNS for full backwards compatibility. Set to 0 to disable DoH.
(default: none) set the URI for your DOH server. That's the URL Firefox will issue its HTTP request to. It must be a HTTPS URL. If "useGET" is enabled, Firefox will append "?ct&dns=...." to the URI when it makes its HTTP requests. For the default POST requests, they will be issued to exactly the specified URI.
Publicly announced servers include: