XUL was ahead of its time when it was introduced by Netscape/Mozilla many years ago. It was a capable XML based language used to describe rich graphical user interfaces. Together with XULRunner this was supposed to be a generic framework for creating graphical applications. This was long before HTML became what it is today. Back then people still thought Java would take off on the desktop. I believe that most Mozilla…
For more details there is the famous Spolsky post about things you should never do—rewrite a product from scratch. Mozilla went one further and tried to reinvent GUI programming as well as building a new product.