What is an XPI file? XPI files mostly belong to Firefox by Mozilla. An XPI file is a Firefox Internet browser plugin. The file is saved as a ZIP archive containing HTML markup, stylesheets (CSS), JavaScript (JS) code, PNG icons, a README.md documentation
An abbreviation for Cross-Platform Install (or XPInstall), a file with the XPI file extension(pronounced "zippy") is a compressed installation archive used to extend the functionality of Mozilla products like Firefox, SeaMonkey, and Thunderbird.