XUL is the same XUL both in extensions and in browser ui. XPCOM is another independent technology https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM h

XUL is the same XUL both in extensions and in browser ui. XPCOM is another independent technology https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM. They are disabling the direct access from extensions to apis used by browser itself (that is access to XUL _and_ XPCOM), to be able to modify the browser ui without worrying about breaking extensions, with the goal to remove XUL eventually.