consider gif2apng conversion
Example of bandwidth savings:
http://littlesvr.ca/apng/gif_apng_webp5.html GIF = 43 132 bytes APNG = 30 823 bytes
Such lossless conversion is better for cartoonish non-video-like GIFs.
Good idea, I didn't even know Firefox supported APNG.
While APNG is a nice format, keep in mind that Janus could be accessed by other browser, which don't support APNG. I think detecting Firefox should be involved when adding GIF2APNG.
Such experimental features should always be opt-in activated via the add-on, so there wouldn't be compatibility issues. There, however, more alternatives to this, which need to be considered.