Not entirely sure, I may go ask Merlin on IRC later. I just enabled that feature and also enabled DoT/Cloudflare for the network apart from son's devices which they are locked to CleanBrowsing Family DNS.
https://www.asuswrt-merlin.net...
Asuswrt-Merlin 384/NG Changelog
384.14 (14-Dec-2019)
- NEW: Implement option to prevent Firefox's automatic usage of DoH.
By default, this will only apply if you have DNSPrivacy
enabled, or if you have DNSFilter enabled with a global
filter, to ensure that Firefox will not bypass either of
these. You can also have this override applied all the
time, or completely disable it.
One has to look out for engineers -- they begin with sewing machines and end up with the atomic bomb. -- Marcel Pagnol
