.XPI File Extension
File TypeMozilla Installer Package
This file is saved in a binary format, which requires a specific program to read its contents.
Compressed installation archive used by various Mozilla applications, including Firefox, Thunderbird, and SeaMonkey; may contain the primary components installed with the software, or may contain an software extension, such as a Web-browser plug-in.
XPI files can often be installed by simply dragging and dropping the file onto an open Web browser window. The Mozilla software uses a component called XPInstall, (which includes Mozilla installer script) to install the XPI packages.