The Free On-line Dictionary of Computing (19 January 2023):

XML User-Interface Language
XUL

    (XUL) An XML-based language created for the
   Mozilla browser for development of cross-platform user
   interfaces.  XUL supports input controls such as
   textboxes and checkboxes, toolbars, menus, dialogs,
   trees, keyboard shortcuts, and more.

   XULPlanet (http://xulplanet.com/).

   (2003-06-14)