[buro9]

This.

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).