"XUL (XML User Interface Language) is Mozilla's XML-based language that lets you build feature-rich cross platform applications that can run connected or disconnected from the Internet. These applications are easily customized with alternative text, graphics and layout so they can be readily branded or localized for various markets."

                             -- DevMo Wiki

We do custom XUL apps whether Standalone or as part of a Firefox / Thunderbird Extension. Our experience lies in working over the course of two years with a third party company a fully functional CRM system built from scratch. 

There are also some examples and tutorials at Joseph Guhlin 's (the CEO) blog:

• XulRunner and Rails(Part 1) 
  
• XulRunner and Rails(Part 2) Connecting the Two Technologies
• Project Management Tutorial Part 1, Planning and Design
  
• Project Management Tutorial Part 2, First Iteration 

Please note that while no tutorial comes to completion both are plenty to get you started. Also note each one uses Ruby on Rails whilst the CRM project used PHP . Xul can work with just about any backend technology, although we specialize in PHP and Ruby on Rails for backend technologies. Please note that Drupal and CiviCRM are also in PHP.

Xul defines the visual style of the application. The rest of the work is done with JavaScript and potentially a backend server and database interactions. To see an example of what can be done with Xul, visit this page with Firefox. Remember that each Xul application can run within or completely separate from Firefox(Firefox not required) by means of a XulRunner Application. With this you can take your web application to the next level as a standalone desktop app.