Added Firefox ebuild, which can use EGL/GLES for WebGL

Also see https://bugzilla.mozilla.org/show_bug.cgi?id=788319

WebGL works mostly fine on x86 desktop systems without libGL.so
when Firefox is emerged from this ebuild (with "egl" USE flag).
The same Firefox patch also enables WebGL when Firefox is
crosscompiled for ARM, though for some reason with heavy CPU
usage in libMali.so on Mali400 hardware.

But somehow WebGL fails when firefox is emerged on ARM from this
ebuild. Weird, isn't it? That's something that still needs to be
investigated.

Some WebGL pages:
    http://learningwebgl.com/lessons/lesson05/index.html
    http://webglsamples.googlecode.com/hg/aquarium/aquarium.html