XUL: XUL Explorer

"XUL Explorer is a XULRunner application that provides an easy way to experiment with XUL. It’s a simple editor that can preview XUL inline or in a separate popup window. It has a list of code snippets (small fragments of XUL or JavaScript) that can be quickly inserted into the editor. The XUL can be loaded from and saved to files. A XUL validator and the Error Console are both available to help debug problems. The help menu provides access to XUL information on MDC. There is even simple “keyword” help lookup for XUL elements. " developer.mozilla.org