Most of those sites are doing a little move called "lying". I occasionally (once every couple of months) run into a site claiming to not support Firefox. I can't recall a single site that wasn't a tech demo of some bleeding edge feature of Chrome that didn't magically start working when I turned on my Chrome UserAgent.

(Hey, if you work at Snapchat: fix your shit. Your desktop site is by far the most mainstream website I've come across that lies like this)