XULRunner is a single "gecko runtime" installable package that can be used to bootstrap multiple XUL+XPCOM applications that are as rich as Firefox and Thunderbird. It will provide mechanisms for installing, upgrading, and uninstalling these applications. XULRunner will also provide libxul, a solution which allows the embedding of Mozilla technologies in other projects and products.
WWW: http://developer.mozilla.org/en/docs/XULRunner