I am using DNS over https and have firefox 63 setup as follows
-no proxy
-Operating DNS system set up with a fake non responsive DNS value
-The URL for DOH- https://mozilla.cloudflare-dns.com/dns-query
When I set Firefox to use network.trr.mode to a value of 3 which I understand to mean use only the DOH DNS values and do not fall back to any other, firefox fails to to resolve any sites. This worked prior to Dec-7-2018 but not anymore. Can you offer an explanation?
Mine works.
I have 63.0.3 on Mojave with trr.mode set to 3.
I also have trr.uri set to https://1.1.1.1/dns-query
True, with the IP it does work.
And it makes sense after all, using a hostname for DNS resolution cant really work 
I have network.trr.uri set for https://mozilla.cloudflare-dns.com/dns-query which is the default for firefox. Does yours work using the default?
Thanks. I have posted to mozila site https://support.mozilla.org/en-US/ as well. I thought this strange as it doesn’t allow evaluation of the network.trr.uri =https://mozilla.cloudflare-dns.com/dns-query alone as the other values will fall back to the OS DNS.
The firefox post is named DNS over https stops working"
Surprisingly, yes, it does.
Mode is still 3, and the url is now the Mozilla one. I restarted Firefox and loaded up bbc.com then checked my about:networking screen for DNS and everything is still TRR=true.
And now we have a mystery. Granted, my Mac is set to use 1.1.1.1 for DNS, so that might have something to do with it.