Ajaxian notified that one of many new features added to Firefox 3 is the support of Animated PNG.
The PNG image format is now
widely accepted as a superior replacement for static GIF images, but
for animated GIFs there has not yet been a clear successor. The new
Animated PNG format (APNG) is a simple extension to PNG, making it
superior for animations too.
Here are some examples that you can run in FF3: