Sniffing your browser?  It really doesn't care much about your browser other than whether or not it is capable of some simple string manipulation and math.  It's the most basic level of defense against trivial DDoS's.
( It's sufficiently basic that a bot I wrote happily grabs the challenge, solves it, submits it, gets the cookie, re-accesses with the cookie, and grabs the data, then re-uses that cookie until the site decides the cookie expired and it starts over again.  I'll have to see how it fares with bitmain's site. )
You're sending vastly more information to a site about your browser just accessing a page in the first place.