Mozilla has disabled automatic selection of DNS over HTTPS providers during a test to enable wider deployment in a more controlled way. Firefox 77.0.1 was released yesterday with an increased rollout of the Web Render feature to more Windows 10 laptops and an updated about:certificate page. Mozilla announced in September 2019 that they would be enabling DoH by default in the Firefox browser, using Cloudflare’s DNS provider as the default DoH provider which led to concerns about the Firefox users’ data being controlled by a single company.
Source: https://www.bleepingcomputer.com/news/security/firefox-7701-released-to-prevent-ddosing-doh-dns-providers/