xpi

XPI is the file extension used by Mozilla-based applications, such as Firefox, Thunderbird, and SeaMonkey, to package add-ons and themes. The name derives from XPInstall, Mozilla’s original extension-installation framework. An XPI file is a ZIP archive that contains an add-on’s code, resources, and a manifest. Older packages commonly include install.rdf, while newer WebExtensions-based add-ons use manifest.json and include the necessary WebExtension files (such as background scripts, icons, and locales). The archive may also contain directories like chrome/ or locale/ for legacy add-ons.

XPI packaging is cross-platform, designed to work on Windows, macOS, and Linux. Users typically install an XPI through the browser’s Add-ons/Extensions interface, or by dragging the file onto the browser window, provided the add-on is signed and approved for distribution. Mozilla’s current policy emphasizes signing for add-ons distributed via its official site, with development builds sometimes allowing unsigned packages in a private testing environment.

In practice, XPI remains the packaging container for Mozilla extensions, even as the platform has shifted toward WebExtensions. This means that, while the underlying API for extensions has evolved, the distribution mechanism (a packaged .xpi file) continues to be a common format for sharing and installing add-ons across Mozilla applications.

Security considerations include downloading XPI files only from trusted sources, as with any software package. Unapproved or tampered packages can pose risks, underscoring the importance of platform-enforced signing and source verification.