Since it knows the DNS will always be public, it also knows that the 192.168.10.20 address is not routable on the internet where it found it.

That is in fact not it. I left the default firefox DNS setting. I simply enabled network.trr.allow-rfc1918 from within the about:config which allows the resolution of local IP addresses. It now works. All my DNS are public, I make no use of any private, local DNS.