The preference network.trr.mode defines the status of TRR in Firefox.
A value of 0 means that it is disabled and not used.
A value of 1 that Firefox uses either native DNS or TRR depending on which is faster.
A value of 2 uses TRR by default but will fall back to the native resolver if the name resolve fails for whatever reason.
A value of 3 enables TRR only mode. Only TRR is used and there is no fallback.
A value of 4 runs it in shadow mode which means that TRR is run in parallel for gathering data but that the native resolver is used.