XUL

XUL (pronounced zool ([zu:l])), the XML User Interface Language, an XML user interface markup language developed by the Mozilla project, operates in Mozilla cross-platform applications such as Firefox. The Gecko layout engine provides the only complete implementation of XUL.

XUL relies on multiple existing web standards and 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 and web-developer documentation is available online at the Mozilla Developer Center.

XUL has no formal specification or interoperable non-Gecko implementations. However, its implementation (Gecko) is open source, tri-licensed under the GPL, LGPL, and MPL.

Mozilla provides experimental XULRunner builds to let developers build their applications on top of the Mozilla application framework and XUL in particular.

As its main benefit, XUL provides a simple and portable definition of common widgets. This reduces the software development effort in a way analogous to the savings offered by 4GL tools.

Reference:
http://en.wikipedia.org/wiki/Xul