All preferences for the DNS-over-HTTPS functionality in Firefox are located under the "network.trr" prefix (TRR == Trusted Recursive Resolver). The support for these are targeted for shipping in release Firefox 62.
set which resolver mode you want.
0 - Off (default). use standard native resolving only (don't use TRR at all)
1 - Race native against TRR. Do them both in parallel and go with the one that returns a result first.