XPI - Mozilla Installer Package
These are compressed installation archives used by most Mozilla applications such as Firefox, SeaMonkey and Thunderbird. They can contain the initial components installed with the software package or can contain software updates or extensions such as web browser plug-ins or add-ons. These files may be installed by simply using a drag and drop onto the open web browser window. These files are installed by a Mozilla component called XPInstall wich installs the XPI packages via an installer script.