A Good Resource for Learning XUL & Javascript?

RJabelman asks: "I'm trying to write a Mozilla extension, but I can't find a decent resource to learn from. Tutorials abound for packaging up an extension, and the web's littered with Javascript snippets to pretty up a web page, but there seems to be very little authoritative information for doing serious work with Javascript, XUL and Mozilla (and more specifically, manipulating XML). I can find my one true resource for every other language or API I've learned: but not this. Can anyone point me to theirs?"