Check out this article: Mozilla News Article on Amazon's use of XUL

XUL and/or XSLT will bring the web over from a content display platform to an applications development platform. web browsers will be application execution platforms. java showed us the way with bytecodes and a virtual machine. XML gave us a standard way to format data. XML is the new 'text file'. XUL is the licence and patent free to write applications that are transmitted over the web. Now a URL will represent an application instead of a web page. When you change something, the UI widget reacts immeadiately instead of waiting for an entire page load.