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XPI (Cross-Platform Install) is Mozilla Firefox's extension package format — a ZIP archive containing the extension's code, assets, and metadata. The name dates back to Mozilla's XPInstall technology from the Netscape era, though modern Firefox extensions use the WebExtensions API and are structurally very similar to Chrome extensions.
Inside an XPI file, you'll find manifest.json (the same WebExtensions manifest format Chrome uses), JavaScript files, HTML, CSS, and icons. Firefox extensions can modify web pages, intercept requests, manage tabs, store data, and communicate with native applications. The WebExtensions API is largely compatible with Chrome's extension API, so many extensions work on both browsers with minimal changes.
Firefox extensions are distributed through addons.mozilla.org (AMO), where Mozilla reviews them for security and policy compliance. Unlike Chrome, Firefox still allows side-loading extensions and supports unsigned extensions in Developer Edition and Nightly builds. This openness is valued by developers and power users but means that malicious XPI files from untrusted sources pose a real risk.