Yelp currently depends explicitly on www-client/mozilla-firefox, which causes problems if one uses mozilla-firefox-bin instead.
yelp cannot be built against firefox-bin, only firefox.
*** Bug 216263 has been marked as a duplicate of this bug. ***
Why must yelp have firefox? In my 64-bit environment, when firefox-bin is replaced by firefox, this breaks plugins which depend on the 32-bit nature of firefox-bin.
Because firefox-bin does not provide the needed libraries and headers to build yelp. Simple.