It's a pretty standard Firefox installation (I also tried disabling uBlock and resetting my privacy settings), and I just confirmed the user agent looks normal, Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:60.0) Gecko/20100101 Firefox/60.0. I think it broke when I moved last week from one EU country to another. Why that would make a difference, I don't know, but Adidas is now serving me WEBP images in Firefox 64 regardless of my browser settings, whether I clear cookies, use a private window, or choose different regional versions of the website.

So, I'm left scratching my head and using Chrome to browse the site.