I’m sorry but I don’t see how that check is browser-specific. Is that part happening on the browser side?
They don’t need to put incriminating “if Firefox” statements in their code – the initial page request would have included the user agent and it would be trivial to serve different JavaScript based on what it said.
Easy enough to test though. Load the page with a UA changer and see if it still shows up when Firefox pretends to be Chrome
The video in the linked article does just that. The page takes 5 seconds to load the video, the user changes the UA, they refresh the page and suddenly the video loads instantly. I would have liked to see them change the UA back to Firefox to prove it’s not some weird caching issue though