How do I set up an XUL document to be the front end of a application using Gecko or XUL Runner?? Or is there no way to do it? Could someone provide an brief example if it is possible? Or link to a website?
From what I've read, it can't be a standalone application, and it involves using XPCOM.
Creating XPCOM Components from the Mozilla developers' network seems to be quite a good guide. It walks you through creating a XPCOM component with every step. That guide might be a bit hard to understand though, and Creating a C++ XPCOM Component from iosart.com might be easier
Firefox addons developer guide / Using XPCOM—Implementing advanced processes and XUL Tutorial / XPCOM_Interfaces, also from the developers' network, explain how to use an XPCOM component inside XUL
You can then run the program by calling XULRunner on the XUL