Why is mode 3 required on Firefox?
The TTR mode 3 of Firefox guarantees the browser won’t fallback on system DNS when a response cannot be obtained through DNS over HTTPS. Because NextDNS blocks some DNS requests based on the blocklist you configured, other modes like mode 2 would make those blocks ineffective and would leak some DNS traffic.
This can be changed by going to about:config and changing network.trr.mode from 2 to 3. Should probably be in the setup details.