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.