Using stable Firefox 67.0.4 64-bit and this is right there on the options page in General/Network settings. Truth to be told, it will set network.trr.mode to 2,

Using stable Firefox 67.0.4 64-bit and this is right there on the options page in General/Network settings. Truth to be told, it will set network.trr.mode to 2, which falls back to normal DNS if anything is wrong, but nonetheless it's there.