I am writing an XUL application. It is not a Firefox extension but a standalone app to be used through XULrunner. My intention is to adopt TDD in my development process and I am
You could try to have a look at Mozmill.
It is an addon that can be used for testing all Gecko based applications.
You can use it to test UI interface of your application.
The documentation is quite ok and if you have any question you can try the dedicated mailing list.