Wikipedia
XUL , which stands for XML User Interface Language, is a user interface markup language that is developed by Mozilla. XUL is implemented as an XML dialect; it allows for graphical user interfaces to be written in a similar manner to Web pages.
XUL can be used to write cross-platform applications such as Mozilla Firefox, where it is interpreted by the layout engine, known as Gecko, which renders Firefox's user interface and Web page display.
In August 2015, Mozilla announced that the use of XUL for creating Firefox add-ons would be deprecated in the future in favour of WebExtensions. Pale Moon, a fork of Firefox, will continue to support XUL indefinitely.
Usage examples of "xul".
Amhantar who wanted the minerals and gems that abound on Utuk Xul and Rabushu and they were willing to kill to mine those treasures.