In the past, there were several toolsets for developing Firefox extensions, but as of November 2017, extensions must be built using WebExtensions APIs. Other toolsets, such as overlay add-ons, bootstrapped add-ons, and the Add-on SDK, are no longer supported.
Extensions written using WebExtensions APIs for Firefox are designed to be cross-browser compatible. In most cases, it will run in Chrome, Edge, and Opera with few if any changes. They are also fully compatible with multiprocess Firefox. You can see the APIs currently supported in Firefox and other browsers.
In 2020, Mozilla will release a new Firefox for Android experience. This new, high-performance browser for Android has been rebuilt from the ground up using GeckoView, Mozilla’s mobile browser engine. We are currently building support for the WebExtensions API on GeckoView.
If you maintain a legacy extension, such as an XUL overlay, bootstrapped, or Add-on SDK-based extension, you can still port it to use WebExtension APIs. There are some porting resources on Firefox Extension Workshop, our site for Firefox-specific development resources.
For more information about transition support, please visit our wiki page.
Addons.mozilla.org, commonly known as "AMO," is Mozilla's official site for developers to list add-ons, and for users to discover them. By uploading your add-on to AMO, you can participate in our community of users and creators and find an audience for your add-on.
You are not required to list your add-on on AMO, but your add-on must be signed by Mozilla else users will not be able to install it.