A bit of nostalgia: I created an application based on XULRunner at the end of high school. The project started out as a Firefox Extension, and since I wanted to create a standalone application, XULRunner was the obvious choice. I didn't know much programming, but it was, coupled with the excellent documentation on MDN, enough to get me started. What I remember best are the following few things:

- XULRunner had an included application update mechanism. You basically created a diff between two application versions and placed that on a server. XULRunner would automatically detect it and apply it. No configuration needed. This was huge.

- XULRunner obviously had a built-in browser. Using a few XML elements, you could embed a browser view.

- Porting from a Firefox extension to a standalone application was very easy. Add some boilerplate and you are done. Pretty much no need to change the application code.

- Since my application didn't include any native code, it was automatically cross platform. And it integrated beautifully into all platforms.