Big changes afoot for Firefox.
We intend to move Firefox away from XUL and XBL, but the discussion of how to do that is in the early stages. There are a ton of unanswered questions: what technologies/best practices for web development should we adopt in its place? How does this affect add-on developers? Is there space for a native-code main-window on desktop like we have on Android? How much time should we spend on this vs. other quality issues? What unanswered questions have we not asked yet?
This clearly isn’t a small endeavour, but the rationale given seems sound to me.