Now that Mozilla is no longer overshadowed (silenced) by Netscape or AOL but is a free-standing independent project the marketing drive is on to get everyone to use Mozilla.

The Mozilla site now sports a Why? special series to promote all the goodies Mozilla offers the world - from grandma to hard-core kernel hackers.

For example, the "Why Use Mozilla as an Application Framework?" page includes a "Did you know?" sidebar stating:

• The Mozilla app framework can be used to build standalone desktop applications like Komodo, apps that run on top of the Mozilla application suite like mozblog, and web applications like the Mozilla Amazon Browser.
• Mozilla has been ported to more than a dozen platforms including Windows (95, 98, NT, 2000, XP, CE), MacOS X, Linux, Solaris, BeOS, OpenVMS, OS/2, AIX, and HP-UX.
• You can build simple but powerful Mozilla-based applications using just JavaScript and XUL.

More @ http://mozilla.org/why/framework.html

And the "Mozilla Application Framework in Detail" page outlines the many benefits XUL offers by stating:

What this means to you as the developer is this: you can take advantage of skills you already have with XML or web technologies to design and implement anything from a simple text editor to a comprehensive IDE - complete with all of the interface widgets that you would find in virtually any major application framework.

Another benefit of this standards-based approach to UI development is that your application is cross-platform "out of the box". Imagine not having to re-write your application three times, or not supporting a less popular platform simply because you do not have the resources for parallel development!

Unlike many other application frameworks, you are not limited to the widget set we provide, nor limited to the "look and feel" of the native OS. You can create applications using our framework that either have a native look and feel for each OS, or one which is identical on Macs, PC's or Unix operating systems.

You may also further enhance the user interface by allowing 3rd parties to develop "themes" for your application. Themes are simply collections of images and CSS which can augment or replace your current UI elements.

Our platform also can take advantage of other internet standards such as XSLT and RDF. RDF, a core element to the framework, allows you to define dynamic elements in your UI (elements that may change after you have completed the application, such as a "history" menu item). XSLT could be used to translate information from web services, RSS, SOAP, or other XML-based languages and convert them into a form that you might display in your user interface.

More @ http://mozilla.org/why/framework-details.html