- There is currently no official MNG support in Firefox of any kind
- APNG has been rejected as extension to PNG and is no w3 recommendation
- Bug #374225 ("make unknown image formats find an extension to display them") is now RESOLVED/WONTFIX.
- The only (widely) supported format for animated raster graphics is GIF, which is patented (in most places outdated now), inefficient, only knows full transparency, only palette colors and is more some kind of 20 year old dirty hack than a file format.
- In the past 15 years, the w3c has not recommended a file format with the capabilities of MNG (or JNG)
IMO it is time to leave the 1980s behind. With or without the w3c. With or without Firefox.
I suggest reopening Bug #18574
, as it is currently VERIFIED/WONTFIX.