Oblivious DNS should be available as a fallback for "Custom"

[tigerhawkvok]

The use case is straightforward, if perhaps edge -- I run a Pi-hole, so my default lookups are already configured with preemptive ad blocking and the like.

But, if for whatever reason, the Pi isn't resolving (container crashed, Pi has an error, etc), as-is, Waterfox won't fall back to any other DNS when I have custom set. Here, I'd like to have a fallback of Waterfox's otherwise excellent oblivious DNS default.