In June Mozilla announced that they were performing a limited Shield study for Nightly users to monitor the performance of DNS-over-HTTPS (DoH) in Firefox. This study uses Cloudflare’s DNS service to encrypt both the requests and responses to any DNS queries in order to increase a user’s privacy. DoH is still being tested by Firefox, but if you want to start using it immediately you can enable it in the about:config settings. To test whether you are now using DoH to resolve DNS queries, you can go to the Browsing Experience Security Check page and click the “Check my browser”” button.”
Source: https://www.bleepingcomputer.com/news/software/mozilla-firefox-expands-dns-over-https-doh-test-to-release-channel/