XUL
XUL ( ZOOL), 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.
Design
XUL relies on multiple existing Web standards and Web technologies, including CSS, JavaScript, and DOM. Such reliance makes XUL relatively easy to learn for people with a background in Web programming and design.
XUL has no formal specification and does not inter-operate with non-Gecko implementations. However, it uses an open-source implementation of Gecko which Mozilla tri-licensed under the GNU GPL, GNU LGPL, and MPL until Gecko changed to GPL-compatible, MPL 2.0.
