James Strachan (of Jaxen and dom4j fame) and Martin van den Bemt launched the Xulux project (http://xulux.org). James writes in his weblog (http://james.weblogger.com):

The Xulux (pronounced zoolooks) project is gathering steam. Its going to be an ASF licenced open source XUL framework. The aim is to simplify the development of rich/thick clients using XUL to define more traditional, rich user interfaces. The XUL can then be transformed into HTML / DHTML / JavaScript for old HTML browsers such as IE while still supporting rich in XUL browsers such as Mozilla or Flash (thanks to ZULU). So the aim is to build a HTML, Swing and SWT clients for XUL as well as a server side framework for developing XUL applications.

So rich clients (Swing, SWT, Mozilla, Flash) can be supported while still using XML and web application development technologies.

It looks like the jXUL guys, who've stopped maintaining jXUL are happy for Xulux to adopt the codebase, ASF licence it and improve it. They may even join Xulux and help out at some point in the future.

Check out their logo. No code yet.