Firefox libraries are put into /opt/firefox. That directory should be included in LDPATH (some apps like Yelp and Liferea depend on its libraries). Maybe in ebuild in src_install() add something like: dodir /etc/env.d echo "LDPATH=${MOZILLA_FIVE_HOME}" > ${D}/etc/env.d/80firefox-bin
Yelp and all other ebuilds use mozilla-firefox or seamonkey, *not* firefox-bin. You can't compile anything against -bin stuff.