In my case, because I have full color management enabled, the AVIF image in the article displays incorrectly (too much saturation on my wide-gamut screen) while the JPG and WebP display correctly.
Firefox AVIF experimental support is disabled by default but can be activated via "image.avif.enabled" flag.
It's worth pointing out that (as far as I know) the fact that canvas elements and videos aren't color managed in Firefox hasn't been considered a blocker for shipping either of them. There hasn't been any movement on the bug I linked in many months (other than users confusing the issue in comments), and so I hope the problem isn't eventually swept under the rug.
I believe this may affect the images in the article, actually. It looks to me like the dark areas of the animal's hair are crushed pretty badly - not just the saturation issue I'm talking about.