One thing we (Mozilla) should do is make it clear that any upstream patch approved for a branch landing (but perhaps not yet landed on that branch, or shipped in a release off that branch) is fair game for trademark-using distributors to apply to their own copy of that branch without further approval. This would help grease the wheels.
The MNG vs APNG issue is a joke. MNG is a huge bloaty spec that has never been implemented by any mainstream browser. It's grotesquely over-engineered --- it includes stuff like scaling of image chunks and linear gradients in its "image format". APNG is a much lighter extension to PNG. We submitted patches for APNG to libpng but the libpng owners rejected them because they like MNG. APNG is implemented in Opera as well as Firefox.
I think we would love people to submit patches to upstream to enable use of system libffi and other system libraries. As far as I know that hasn't happened yet.