"XUL overlays are mostly obsolete" is complete crap. They're used by most extensions and there's nothing wrong with them. Please point me to a discussion where somebody gives a good reason they should be considered obsolete.
MDN is changing to focus just on documenting web technologies. All the same great content is still here; we’re just changing some visual elements and navigation, to help you more quickly find the web technology docs you’re looking for.
But don’t worry, MDN and Mozilla are still together! In fact, we're updating MDN’s look to reflect Mozilla's new logo and colors.