You must Disable Firefox's DNS-over-HTTPS aka Trusted Recursive Resolver
Firefox for some time has been experimenting with something called DNS-over-HTTPS. It's a massive change to one of the most fundamental mechanics of the Internet and could have some seriously devastating effects. Since Firefox 74 it's enabled by default for new users, and since Firefox version 79 existing users that have up till now not enabled it are being asked to opt-in using a misleading popup.