Currently it is not easy to distinguish against what headers to build programs like epiphany/galeon or mplayerplug-in. We have 3 Packages in the tree that do it: gecko-sdk,mozilla,firefox To save space and compile-time and to have always the latest sources forefox/mozilla should use the gecko libraries and includes provided by gecko-sdk or a new yet to be created package for gecko and dev-libs/nss and probably some other libs.
You could look here in order to understand, why this is impossible for now. the mozilla bug four years old - https://bugzilla.mozilla.org/show_bug.cgi?id=53727 and the debian comments on their bugzilla - http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=265626
this is probably not possible to do, sad :(