mozconfig-v6.55.eclass sets: RDEPEND="... >=media-libs/libpng-1.6.28:0=[apng] ..." But old-configure.in sets: MOZPNG=10629 So dependency in eclass should be updated.
commit ce70638addd8c3abb48167baf6dc9f70d3e01b7e (HEAD -> master, origin/master, origin/HEAD) Author: Lars Wendler <polynomial-c@gentoo.org> Date: Thu Aug 17 11:28:44 2017 mozconfig-v6.55.eclass: Adjusted libpng dependency (bug #628088).