Firefox is the only ebuild that I have on my system to require mng. I have been building Firefox for a while now without libmng being on my system. I never see any mng's on the internet so I have no use for the lib. I suggest using the mng use flag to make it optional. Reproducible: Always Steps to Reproduce:
Someone might solve really old Bug 54651 at the same time. ;-)
*** This bug has been marked as a duplicate of 111733 ***