The Firefox UI Is Now Built with Web Components
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 products such as the Mozilla suite, Thunderbird and Firefox used this for a very long time.<p><a href="https://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" rel="nofollow">https://www.mozilla.org/keymaster/gatekeeper/there.is.only.x...</a>