TeachingMachines writes "A stable developer preview release of XULRunner 18.104.22.168 is now available. Based on the Firefox 22.214.171.124 codebase, it is available for Windows, Mac OS X, and Linux. From the Mozilla Developer Center (beta): "XULRunner is a Mozilla runtime package that can be used to bootstrap XUL+XPCOM applications that are as rich as Firefox and Thunderbird. It will provide mechanisms for installing, upgrading, and uninstalling these applications. XULRunner will also provide libxul, a solution which allows the embedding of Mozilla technologies in other projects and products." Help with programming with XUL and its related technologies can be found at XULPlanet. Beginning programmers will benefit especially from the XUL Tutorial. Also check out the XUL Element Reference to get an idea of what's available. "
A couple of other resources are worth mentioning. First, there is the XUL Programmer's Reference Manual which covers interface elements for XUL version 1.0. "Rapid Application Development with Mozilla" is available for download at Bruce Perens' Open Source Series page. If you get the book, make sure to check out the errata. Unfortunately, the author Nigel McFarlane has passed away, so this is likely the final version. One final reference, "Creating Applications with Mozilla," is available here.
For those individuals who are looking for an extremely powerful application framework that is relatively easy to use, Mozilla is definitely worth a look.