Open firefox and type:
Probably you already have:
If not add it or use something like:
network.trr.mode by setting it to
network.trr.mode to 2 make DNS Over HTTPS the browser's first choice
but use regular DNS as a fallback (0 is "off by default", 1 lets Firefox pick
whichever is faster, 3 for TRR only mode, 5 to explicitly turn it off).