XUL Mozilla firefox programming

Mozilla Firefox is a free and open source web browser developed for Windows, OS X, Linux, and Android by Mozilla Foundation and its subsidiary, the Mozilla Corporation. Firefox uses the Gecko layout engine to render web pages, which implements current and anticipated web standards. Copied from wikipedia

 Have you worked with FireBug and other application that works over firefox. XUL (XML User Interface Language) is Mozilla's XML-based language for building user interfaces. Official site can be found at, https://developer.mozilla.org/en/docs/XUL

See this, http://getfirebug.com/ Firebug and other firefox's extension are developed in XUL language.
Firefox extensions are applications that add new functionality to Mozilla applications such as Firefox and Thunderbird. It is also worth noting that there are differences between the definition of extension and add-on. All extensions are add-ons, but add-ons can also be themes, plugins, or language packs. XUL uses, mixture of XML, HTML, JS, CSS, Resource Description Framework (RDF),  etc. So, if you are web-designer/developer you will feel at home. More coming soon.

XUL (pronounced "zool") is one of many technologies used for creating Mozilla-based products and extensions. It is only one part of the development landscape, but given that it's practically exclusive to Mozilla, it tends to be used to identify all Mozilla-related development.  Copied from https://developer.mozilla.org/en-US/docs/XUL/School_tutorial/Introduction (also official site of firefox for learning XUL). More coming soon.