If you’re using an earlier version of Impromptu you should look at downloading this new version(it should be backwards compatible unless you’ve made changes).  Previously Impromptu worked great in all browsers except IE6.  Why IE6?  Because of its lack of support for position: fixed; After beating my head on my desk for hours it finally come together.

Previously in IE6 it would fade out the screen but the prompt would stay put, sometimes causing it to be scrolled out of view.  Now that is all fixed. Be sure to check the css on the project page.  A previous IE css hack has been removed and Impromptu takes care of all cross-browser issues just like it should :)
Download Impromptu 1.2