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.