The rollout will take place over the course of several weeks to ensure stability, but once it’s finalized, all Firefox users running the latest version of the browser will have DNS protection. In short, DoH functionality helps to obfuscate and encrypt DNS lookup requests, which can otherwise expose information like the websites you visit.
Obviously, there’s nothing inherently malicious about DNS lookups — they’re an essential part of the internet as we know it. Mozilla explains the technology as follows:
