In the Google Play Store Firefox Install Nightly for Developers
Open Firefox Nightly and enter about:config in the URL address bar. Then edit the following value:
network.trr.mode = 3
The following values are possible:
0 - Default value in standard Firefox installations (currently is 5, which means DoH is disabled) 1 - DoH is enabled, but Firefox picks if it uses DoH or regular DNS based on which returns faster query responses 2 - DoH is enabled, and regular DNS works as a backup 3 - DoH is enabled, and regular DNS is disabled 5 - DoH is disabled
network.trr.uri = https://cloudflare-dns.com/dns-query
Other possible DoH servers are in on Wikipedia.