Firefox Add-ons, at addons.mozilla.org (AMO), is the official catalog of extensions and themes for the Firefox browser, run by Mozilla. An extension is a small program that adds features to the browser. Firefox add-ons are built with the WebExtensions API, an extension interface shared across browsers, so an extension written for Chrome may already work in Firefox.
A developer creates an account on AMO, packs the extension into a ZIP archive (.zip, .xpi or .crx, at most 200 MB) and uploads it in the Developer Hub, with the web-ext command-line tool or through an API. An automatic validator checks the package, and the developer adds a name, summary, description and up to two categories. Mozilla signs the add-on, because release and beta versions of Firefox install only signed extensions. AMO is built into the Firefox Add-ons Manager, and Firefox updates installed copies when a new version is listed.
Listing follows Mozilla's Add-on Policies and review. Mozilla keeps AMO free to download and offers no way to sell add-ons through it.