XUL stands for XML User Interface Language. It is an XML markup language that allows for GUI-based application creation using the Mozilla platform. In turn, the Mozilla platform is composed of several technologies used to build applications within the Mozilla Project. Firefox, Pale Moon and Thunderbird are just three applications built using XUL and other Mozilla project technologies. XUL makes it easy to build your own stand-alone applications through the use of XULRunner, XPCOM, JavaScript, HTML and CSS. This book is intended to be a quick start-style resource to give you the basics of creating a XUL GUI along with how to make that GUI actually do something using JavaScript and XPCOM. While xulrunner has been deprecated as of version 41, this 6th Edition incorporates information on using Firefox and Pale Moon to run your stand-alone applications. Additional expansions on information have been added throughout.