Support for MNG/JNG images and animations was removed from Mozilla after version 1.4. In the following Peter Weilbacher present a few options to again enable MNG/JNG support for Mozilla under OS/2 and eCS. MNG support for other platforms can be found on Christian Biesinger's website (that's where I got the install.js from).
http://weilbacher.org/Peter/mozilla.html
Source: OS/2 News and Rumors