Hi Chen,
For modules/libimg/png, mozilla products use aPNG which was rejected
by upstream.
http://en.wikipedia.org/wiki/APNG
So we have to use internal png.
For media/libvorbis, mozilla has custom patches in the source tree.
https://bugzilla.mozilla.org/show_bug.cgi?id=517422
Fro media/libvpx, I don't know well. However you are welcome to file
the issue to
bugzilla.mozilla.org.
Takanori