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Add-ons add new functionality to Gecko -based applications such as Firefox, SeaMonkey, and Thunderbird.

Modify and extend Mozilla applications

Add-ons add new functionality to Gecko-based applications such as Firefox, SeaMonkey, and Thunderbird. There are two main types of add-on: Extensions add new features to the application, while Themes modify the application's user interface.

For both extensions and themes, Mozilla operates a repository at, also known as AMO. When you submit add-ons to AMO they are reviewed, and after passing review they are made available to users. You don't have to submit add-ons to AMO, but if you do, users can take confidence in the fact that they have been reviewed, and you can benefit from AMO's visibility as a source for useful add-ons.

Add-ons can greatly affect the behavior of the application that hosts them. We've developed a set of guidelines to help ensure that they provide a good experience to users. These guidelines apply for all sorts of add-ons, whether they are hosted at or not.


Extensions add new functionality to Mozilla applications such as Firefox and Thunderbird. They can add new features to the browser, such as a different way to manage tabs, and they can modify web content to improve the usability or security of particular websites.

There are three different techniques you can use to build extensions: Add-on SDK-based extensions, manually bootstrapped restartless extensions, and legacy extensions.


We're working on a system called WebExtensions, which is a new way to develop WebExtensions for Firefox, that will be largely compatible with the system used by Chrome and Opera.

In the future this will be the preferred way to develop extensions for Firefox.

At the moment the implementation of this is experimental, but you can see the docs here if you want to give it a spin.

If you can, it's advisable to use the Add-on SDK, which uses the restartless extension mechanism but simplifies certain tasks and cleans up after itself. If the Add-on SDK isn't sufficient for your needs, implement a manual restartless extension instead.

For more information on choosing which technique to use, read this comparison.


Extension development is hard without being able to debug to see what lines errors were made on. You must enable the developer preferences in order for the logs to be show in the Browser Console. For desktop see here: Setting up an extension development environment.

For desktop follow Setting Up a Development Environment, for Mobile (Android/iOS) follow Debugging Firefox for Android with WebIDE which uses the "Browser Toolbox" from within the desktop WebIDE to catch errors occuring on the mobile device, for Firefox OS also use the WebIDE.

No matter how you develop an extension, there are some guidelines you can follow to help ensure your extension provides as good a user experience as possible.

Ensuring your extension is fast, responsive and memory-efficient.
Ensuring your extension doesn't expose the user to malicious websites.
Ensuring your extension plays nicely with other extensions.


Most of the documentation assumes you're developing for Firefox Desktop. If you're developing for some other Gecko-based application, there are major differences you need to know about.

Developing extensions for the Thunderbird mail client.
Firefox for Android
Developing extensions for Firefox for Android.
Developing extensions for the SeaMonkey software suite.


Themes are add-ons that customize the application's user interface. There are two sorts of themes: lightweight themes and complete themes.

Lightweight themes are much simpler to implement than complete themes, but provide very limited customization.

With complete themes you can make much deeper modifications to the application UI. The documentation for complete themes is out of date, but is linked to here as a possible basis for updated documentation.

Other types of add-ons

Search engine plugins are a simple and very specific type of add-on: they add new search engines to the browser's search bar.

Plugins help the application understand web content that it does not natively support. NPAPI plugins are a legacy technology and new sites should not use them. In general, plugins are not available on most modern mobile systems including, and websites should transition away from using plugins.

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