XULRunner is currently a product in development which provides a runtime environment for XUL applications. Therefore all XUL applications for Mozilla FireFox, Mozilla Thunderbird, Democracy Player and Songbird will be able to executed using this product.
Currently XULRunner 1.8.0.4 is the most stable build so far with XULRunner 1.9 (the first production version) scheduled to be released with Firefox 3 late into 2007.