В статье ИМХО ошибка,

network.trr.mode=0 это дефолт, a не запрет и как только они решат что дефолт DoH, то ноль будет DoH

Для запрета нужо юзать network.trr.mode=5

https://wiki.mozilla.org/Trusted_Recursive_Resolver

network.trr.mode

    0 - Off (default). use standard native resolving only (don't use TRR at all)
    1 - Reserved (used to be Race mode)
    2 - First. Use TRR first, and only if the name resolve fails use the native resolver as a fallback.
    3 - Only. Only use TRR, never use the native resolver.
        Up to FF >= 73, this mode also requires the bootstrapAddress pref to be set.
        Starting with Firefox 74, setting the bootstrap address is no longer mandatory - the browser will simply bootstrap itself using regular DNS, unless the DoH server domain can't be resolved.
        The native resolver will still be used for portal detection and telemetry (Bug 1593873)
    4 - Reserved (used to be Shadow mode)
    5 - Off by choice. This is the same as 0 but marks it as done by choice and not done by default.