Add-ons allow developers to extend and modify the functionality of Firefox. They are written using standard Web technologies - JavaScript, HTML, and CSS - plus some dedicated JavaScript APIs.
Among other things, an add-on could:
There are several types of add-ons, but the most common type are extensions.
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.
If you are writing a new extension, use WebExtensions APIs.
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.
See the APIs currently supported in Firefox and other browsers. We're continuing to design and implement new APIs in response to developer needs.
Most of the WebExtensions APIs are also available on Firefox for Android.
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 MDN.
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.
For an overview for the process of publishing your add-on see, Signing and distributing your add-on.
In addition to extensions, there are a few other add-on types that allow users to customize Firefox. Those add-ons include:
Search engine plugins add new search engines to the browser's search bar.
User dictionaries let you spell-check in different languages.
Language packs let you have more languages available for the user interface of Firefox.
You can use the links below to get help, keep up to date with news around add-ons and give us feedback.
Use the Add-ons Discourse forum to discuss all aspects of add-on development and to get help.
Use the dev-addons list to discuss development of the add-ons ecosystem, including the development of the WebExtensions system and of AMO:
If you're a fan of IRC (Internet Relay Chat), you can get in touch at:
If you discover a security vulnerability in an add-on, even if it is not hosted on a Mozilla site, let us know and we will work with the developer to correct the issue. Please report them confidentially in Bugzilla or by emailing amo-admins@mozilla.com.
If you find a problem with the site, we'd love to fix it. Please file a bug report and include as much detail as possible.