This is almost certainly a firewall rule put in place by the operators of that site. My own sites which are protected with Cloudflare do not exhibit this behavior when using Firefox.
I used to work there, and there wasn't a global "do this for this browser" (except for a little bit to reduce annoyance specifically for the Tor browser).
It is almost 99% certain to be a site operator firewall rule based on the browser user agent. This may even be accidental, they may have been hit by an aggressive bot using a UA string that matches Firefox and the site operator may not even realise they've done this (if they use Chrome, which is likely).