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