After the redesign of the settings in the desktop version of Firefox (I don't remember which version it was), I'm a little confused by the built-in DoH setting. Previously, there was a Maximum Protection mode, which meant ONLY using the selected provider. Now, this is controlled by the "Always warn if secure DNS is unavailable" checkbox. This name immediately made me think that this would simply display warnings, and it wasn't meant to ONLY work with the selected provider. It would be a good idea to rename this checkbox to something like "Always use the selected provider and warn when it's unavailable." Basically, just my opinion…
Source: r/firefox · by /u/AvlasenkoVitaliy