I have been experimenting with EXT JS in XULRunner. I'm using it to build yet another photo upload application, similar to some Firefox extensions that you may have seen such as Fireuploader. I don't have a lot of details to report, however, I really like using the EXT Javascript framework. It's a bit complicated at first but the interfaces that it produces are really slick. Once I have a working demo I plan to post it here as an example of what can be done with XULRunner and EXT. Stay tuned!