Comment 1 for bug 1596678

It seems to be specific to sites requesting Helvetica as their font, facebook and twitter both request "Helvetica, Arial, sans-serif", whereas Google just requests "Arial, sans-serif" and displays all emoji in colour. So I suspect we have some font that's aliased to Helvetica in some way that provides its own implementation of the smiley faces in monochrome which is overriding the emojione font.