worked fine on firefox & linux, the file shows as .webp to me.
Decided to investigate this a bit: when opened to new window, the image url has ?format=webp query argument, if I change that to ?format=jxl then it breaks as the server actually provides a .jxl file. At least I had to TRY to break it :P
The built-in Firefox support is only activated for unstable builds, so you can’t enable it on stable unless you manually enable it during compile-time.