It would be a security nightmare to QAs if people start using XPI for testcases. It would be far better to write standalone XUL, with embeded JS and CSS, that people can read first and then paste to a macro/xul runner editor.

(From Alex: I'd love to know where there is such a xul runner system as you describe it, such that I could create an instance of an XPCOM component such as nsITransactionManager, specifically for testcasing something.)

Alex: Extension Developer extension by Ted Mielczarek includes a XUL editor, which is a textarea you can paste your XUL code to. As you can include js scripts in xul directly, this may be what you need.

see its thread