Yes, it's trivial. It's also very annoying to the probably 99% of users who don't care about it at all, especially if this becomes just one of many settings that needs to be configured on startup.

Make it random then.

Random security settings that differ with each installation? No thanks, I’d rather not play dice with security.

Every question you make a potential user/customer answer is a potential loss of conversion.

Most Firefox users do not care. Those that do can figure out how to change it easily enough.

But Cloudflare also happen to be the fastest DNS resolver.

Fastest isn't necessarily best.

My ISPs caching DNS, and any caching DNS running on IP addresses belonging/advertised by my ISP by BGP to various CDNs, are the best possible responses.

I don't care if the p99 DNS response from my ISP is 50% slower than Cloudflare, if the streaming video, or large download, or many small files requests are better served by CDNs in my ISPs network that are not visible to Cloudflare.

All DNS benchmarks I have seen focus only on the DNS response time, never on the DNS response quality.

But that's because they are mostly written by people who don't know how the internet (or competent ISPs) actually work. Some of them even seem to log errors when they get unexpected responses for some well-known URLs (like google.com) because they don't know there are new Google sites than when they last checked ...