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.
一个XPI含有安装指令（install.js 或 install.rdf)和真正需要安装的软件。这个软件一般是用JAR格式打包的。当XPI包被下载或者拖动到插件管理器之后，XPInstall自动读取XPI里的安装指令，并安装所包含的软件。
When XPI files are served via HTTP, the
application/x-xpinstall MIME type should be matched with the
xpi file extension.