An XPI file is a compressed archive used by Mozilla-based applications like Firefox, Thunderbird, and SeaMonkey to distribute and install extensions, themes, and other add-ons. It's essentially a ZIP archive containing the files necessary for the extension to function, including JavaScript code, CSS stylesheets, images, and an install manifest file (install.rdf). This manifest file provides metadata about the extension, such as its name, version, author, description, and compatibility information. When a user installs an XPI file, the application extracts the contents of the archive and integrates them into the application's directory structure, enabling the extension's functionality. XPI files provide a convenient and secure way to distribute and manage extensions, ensuring that users can easily add new features and customize their browsing or email experience. The format allows for digital signatures, which helps verify the authenticity and integrity of the extension, reducing the risk of installing malicious or compromised add-ons. The XPI format is specific to Mozilla applications and is not directly compatible with other web browsers or software.