The .XPI file type is associated with Mozilla Installer packages. These are nothing but packaged files that are used to install add-on functionality and extensions for the mozilla browser and other applications. Most popular XPI package files are for products like Firefox, Thunderbird and SeaMonkey.

The file structure of these XPI packages closely resembles that of other zip files. These XPI packages allow to customize the Mozilla applications and install new functionalties that improve the productivity for the user.Most of the times, these packages contain a metadata file as well as a javascript file for running the installation code.