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.
You’re right, this is a form of harassment, and it needs to be recognised as such.
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).