XPI
From MDC
Cross-Platform Installer Module (XPI) (pronounced "zippy") is a ZIP file used to install packages, utilizing the XPInstall technology. XPI modules (so called "Bundles") are employed to install a wide variety of software, including Plugins, Extensions, Themes, and Thunderbird dictionaries.
A XPI contains installation instructions (install.js or install.rdf) as well as the actual software to install, which is often itself packaged as a JAR file. When downloaded or dropped into an extension manager, XPInstall automatically interacts with the installation instructions contained in the XPI, and installs the contained software.
When XPI files are served via HTTP, the application/x-xpinstall MIME type should be matched with the xpi file extension.