Youtube. Ria. XUL Application Packaging. XULRunner application packages are standard toolkit bundles (like a Firefox extension), with one additional manifest file (application.ini) which helps describe the application.
XUL app packages are typically ZIPped into an archive with the extension .xulapp or .xpi. They can be installed to XULRunner with a command line flag "--install-app" if XULRunner is registered on the system. Extreme Programming: A Gentle Introduction.