I've seen an XUL-based application recently that supposedly works on the desktop. Isn't XUL the Firefox language? I thought it wouldn't work on anything other that Firefox and certainly wouldn't work as a desktop application.
Can someone who knows more about XUL confirm its compatibility with other browsers (IE, Chrome, etc.) and if it runs as a desktop application, its compatibility with operating systems (Windows, Mac, Linux)
check out http://en.wikipedia.org/wiki/XULRunner. it was created to run xul applications like firefox. songbird is also based in XUL. for more detail see this SO question.
to actually answer the real question, yes, desktop apps can be built in xul, as firefox is. the wiki page i linked to even points to a video game being built using it. at its core its just another application framework.
XUL is a descriptive language for UI used in Firefox but also used in other mozilla apps, like Thunderbird for example
There is a projet named Xul Runner which allow using XUL for desktop apps (see Pencil for example).
AFAIK, all Mozilla apps are compatible with Windows, Mac, Linux and other system for some.
XUL is a user interface language. It was created originally for mozilla/firefox, but is also used by thunderbird.
The Gecko layout engine is what renders XUL and any application that uses this engine can be built with it. See XUL Runner.
The layout engine has been written with cross platform concerns, so it runs on Windows, Linux and Macs.
XUL only works on xulrunner, and Firefox is built on top of xulrunner. Other applications are built on xulrunner (e.g. Thunderbird). You can also build applications on top of xulrunner.