Some spam, tracking and ad servers are blocked at the DNS level on my network. For me it is therefore undesirable that programs use their own DNS.

After looking at https://blog.ungleich.ch/en-us/cms/blog/2018/08/04/mozillas-new-dns-resolution-is-dangerous/

I disabled Trusted Recursive Resolver in Firefox. In about:config:

network.trr.mode = 5

Look at https://gist.github.com/bagder/5e29101079e9ac78920ba2fc718aceec what this code means.