Remember, you can always opt out of sending any technical or usage data to Firefox.
How about you show you respect user privacy by making it an opt-in…?
Feels like no matter where I turn, even the “privacy friendly” options turn away from privacy eventually.
Try using the private IP options instead and see if that works. The generic one being 10.64.0.1, but other options that include ad voicing and such ranging from 100.64.0.1 to 100.64.0.25 or something like that. I’ve got my entire network setup behind their VPN and a a pihole pointing to one of their private DNS addresses without any issues. I left their pubic DNS years ago so that I could make sure my DNS requests were always within the tunnel instead