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I haven't seen anything specifically yet. Resolution itself shouldn't be an issue as Cloudflare should do regular DNS requests on the client behalf, but my concern would be for active/active style load balancing. If everyone using firefox suddenly starts using the "same" local DNS server then that could introduce some uneven load balancing depending on method and how well CF respects TTLs/etc

we (and I'm sure others) use split dns, internally its all different... this forces everything to the internet to decide where to go

There are a few ways you can ask Firefox nicely to disable this behavior outlined in their blogs, but honestly if I was in charge of an environment I'd probably block Cloudflare's DoH (or DoH in general if not in use) address at the firewall. That should force it to silently fallback to normal resolution

thanks for the suggestions!