The spec defines appendChild (and others) on Node. assert(Node.prototype.appendChild) In Firefox different "subclasses" of Node implement these. For example Object.getOwnPropertyDescriptor(Text.prototype, 'appendChild'); Object.getOwnPropertyDescriptor(Element.prototype, 'appendChild'); what is worse is that every single DOM interface that implements Node redefines these. Object.getOwnPropertyDescriptor(Element.prototype, 'appendChild'); Object.getOwnPropertyDescriptor(HTMLElement.prototype, 'appendChild'); Object.getOwnPropertyDescriptor(HTMLBodyElement.prototype, 'appendChild'); and these are not reflected as the same function object assert(Element.prototype.appendChild === HTMLElement.prototype.appendChild) This can be manually fixed by doing something like: Node.prototype.appendChild = Element.prototype.appendChild; delete Element.prototype.appendChild; delete HTMLElement.prototype.appendChild; delete HTMLBodyElement.prototype.appendChild; ...

Yep, known and is fixed for the new bindings.