XPInstall

XPInstall (XPI)
Filename extension
.xpi
Internet media type
application/x-xpinstall
Developed byMozilla Foundation

XPInstall (Cross-Platform Install) was a technology used by the Mozilla Application Suite, SeaMonkey, Mozilla Firefox, Mozilla Thunderbird and other XUL-based applications for installing Mozilla extensions that add functionality to the main application. Support for XPInstall was removed from Firefox in November 2017 with the release of Firefox 57.