User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.13; rv:66.0) Gecko/20100101 Firefox/66.0

Steps to reproduce:

TRR will not work properly for accessing IPv4-only resources in an IPv6-only DNS64+NAT64 environment as the local resolver will not be able to do DNS64.
(This includes many mobile networks such as T-Mobile US, as well as an increasing number of wifi and fixed-line environments such as at tech conferences, enterprises, and universities.)

Actual results:

When a TRR DoH server returns only an IPv4 A record, an IPv6-only client will be unable to connect.

Expected results:

When no AAAA record is available, a TRR DoH client should perform rfc7050 dns64 synthesis. Clients must strongly prefer the native AAAA records when available for names so as to not send unnecessary traffic to the NAT64 gateway.

This seems to be a duplicate of Bug 1589781 which was recently fixed.