.XPI File Format

Firefox add-on package

Introduction

A .xpi file packages an add-on for Firefox. It is a ZIP archive with another extension, as Mozilla’s documentation says. Today’s add-ons are WebExtensions: the archive holds the extension’s files directly, with a manifest.json at the top. That file is the same kind of manifest as a Chrome extension’s.

Add-ons from before WebExtensions have an install.rdf instead. Older still, an XPInstall package has an install.js script. The name comes from the old XPInstall installer.

Structure

A file dropped on any page of this site opens in the explorer, which shows the manifest, the signing files and every member of the archive.

  1. ZIPmanifest.json the extension’s manifest, at the top of the archiverequired
  2. ZIPThe extension’s files scripts, pages, images, _locales/varies
  3. ZIPMETA-INF/ the signatures, in a signed add-onsigned add-ons
A WebExtension add-on: a ZIP of the extension’s files, not of a folder that holds them.

Mozilla’s documentation is explicit that the archive holds the extension’s files themselves, not the folder they are in. Mozilla signs add-ons, and release versions of Firefox install only signed ones.

The rest of the format is described on these pages:

Links

Software

  • Firefox – Mozilla’s browser, which installs add-ons from .xpi files (Windows, macOS, Linux)
  • addons.mozilla.org – Mozilla’s site for publishing and installing add-ons

Developers

  • web-ext – Mozilla’s command-line tool that builds and signs add-ons (Node.js)

Reference