They were disabled in february 2019, as depicted in this thread:
Stop animated images in FireFox / Tor browser / Chrome.
Apparently, they were not intended to be allowed, buy PNGs managed to get away with it until the matter was looked into and promptly handled:
OK, APNGs are now detected. Old ones are grandfathered for now, in part because people have ongoing campaigns, but in the future I might (or might not) go through and remove them.
They were pretty annoying and distracting when reading.