If you want it to just always use what you've configured and not have to follow the changes they make you can set network.trr.mode to 5 in about:config. 5 signifies "off by choice"

If you just want to know how to toggle it normally in case something goes weird with one of the changes it's in the network settings part of the settings page.

see https://support.mozilla.org/en-US/kb/firefox-dns-over-https#...

edit: there is also the canary domain https://news.ycombinator.com/item?id=21825443