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.
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