network.trr.mode is the relevant setting Here is the MozillaWiki article on it:
When it is set to ‘0’ my Firefox works as expected. And is presumably not doing DNS over HTTPS (yet).
I’d rather make sure it stays off even if the default behavior changes.
When it is set to ‘5’, documentation says it should force it off no matter what the default behavior is. In practice though all DNS resolution in Firefox breaks for me.
I am on Firefox 60.6.1esr.