I suspect that people operating Web sites have no idea how many legitimate users are blocked by CloudFlare.
And. based on the responses I got when I contacted two of the companies whose sites were chronically blocked by CloudFlare for months, it seemed like it wasn't worth any employee's time to try to diagnose.
Also, I'm frequently blocked by CloudFlare when running Tor Browser. Blocking by Tor exit node IP address (if that's what's happening) is much more understandable than blocking Firefox from a residential IP address, but still makes CloudFlare not a friend of people who want or need to use Tor.
I sometimes wonder if all Cloudflare employees are on some kind of whitelist that makes them not realize the ridiculous false positive rate of their bot detection.
I've contacted companies about this and they usually just tell me to use a different browser or computer, which is like "duh, really?" , but also doesn't solve the problem for me or anyone else.
I'm guessing if it's really Resist Fingerprinting on Firefox (something Mullvad also has on by default), then there are other settings that aren't being enabled causing the issue. Mullvad actually lists the settings related to resisting fingerprinting here - https://mullvad.net/en/browser/hard-facts