[Mozilla Enterprise] disabling DoH via network.trr.mode

[PE]

I'm testing DoH explicitly on and off with ESR 60.9.0esr.

According to https://wiki.mozilla.org/Trusted_Recursive_Resolver,
network.trr.mode set to 0 might still decide to use DoH, but 5 is
supposed to disable it completely.

However, when I set it to 5, DNS resolution is broken completely.

Is this a bug? If not, what do I set this to so that DoH will never
be tried and DNS queries will only go to the system stub resolver?