During configure phase ebuild fails due missing dependency. cchecking for libffi >= 3.0.9... Requested 'libffi >= 3.0.9' but version of libffi is 3.0.8 Separately emerging to the latest stable version of dev-libs/libffi resolves this problem. I recommend that the maintainers review the dependency in the firefox ebuild.
+ 02 Nov 2011; Samuli Suominen <ssuominen@gentoo.org> libffi-0.ebuild: + Force at least 3.0.9 wrt #389245 by Ross Drummond