The Mozilla team decided to use APNG over MNG (both are extensions to the PNG image format that allow animation). This was discussed in Bug 257197, where it was mentioned that APNG violates section 5.2 of the ISO PNG spect in order to adapt more widespread compliance (read: Allow IE to view it) instead of MNG.

Further reading Edit

External links Edit

MNG 4 IE - Allows IE users to view MNG