edit: I checked it now and I don't actually see any APNGs in the website? Only auto-playing mp4s which Firefox allows because there is now sound, and also some webm. Didn't really notice a pattern on why each one was a different file format and displaying one or the other didn't show noticeable performance impact. JavaScript disabled by the way.
Also, while trying to reproduce I reopnened Firefox process manager but it only listed the website as using 20% CPU which is still a lot but somewhat to be expected with the amount of auto playing videos. Previously the flame graph for the page performance had a huge bar dedicated to "Image Decoding" however I couldn't really find the culprits unfortunately. What I found is that the issue intermittent, it's possible when the videos in the page restart Firefox for some reason had to decode them again and thus a spike in CPU usage.