The Mozilla Extension Package (XPI) is a ZIP-based archive format used for distributing extensions, themes, and other add-ons for Mozilla-based applications like Firefox, Thunderbird, and SeaMonkey. It contains the necessary files for the extension to function, including JavaScript code, CSS stylesheets, images, and manifest files that describe the extension's metadata, permissions, and dependencies. The XPI format allows for easy installation and uninstallation of extensions, providing a convenient way to enhance the functionality and customize the appearance of Mozilla applications. The manifest file, typically named install.rdf or manifest.json, is crucial as it provides information about the extension's name, version, author, description, and the application it's compatible with. Digital signatures can be included within the XPI to verify the authenticity and integrity of the extension, ensuring that it hasn't been tampered with. This helps protect users from malicious or compromised add-ons. The XPI format is a cornerstone of the Mozilla add-on ecosystem, enabling developers to create and distribute a wide range of extensions that enhance the user experience.