[trog]

Firefox user - have seen this many times. Always assumed it's either NoScript or Firefox's built-in tracking protection meaning there's some pre-existing cooking not set that Cloudflare places from other site visits, or because some script is getting blocked somewhere else.

[rezonant]

Seeing the cloudflare challenge as a user can happen in any browser depending on the site and it's configuration. A site can choose a security level that requires all visitors to receive a JS/captcha challenge, and sites can make custom firewall rules to require JS/captcha challenges on any of hundreds of different attributes of a particular request using CF's web application firewall tools. One of those attributes is user agent, for instance.