For

Mozilla Mozilla is a free software community founded in 1998 by members of Netscape. The Mozilla community uses, develops, publishes and supports Mozilla products, thereby promoting free software and open standards. The community is supported institution ...

software Software consists of computer programs that instruct the Execution (computing), execution of a computer. Software also includes design documents and specifications. The history of software is closely tied to the development of digital comput ...

, an add-on is a

software component A software component is a modular unit of software that encapsulates specific functionality. The desired characteristics of a component are reusability and maintainability. Value Components allow software development to assemble software ...

that extends the functionality of the

Firefox Mozilla Firefox, or simply Firefox, is a free and open-source web browser developed by the Mozilla Foundation and its subsidiary, the Mozilla Corporation. It uses the Gecko rendering engine to display web pages, which implements curr ...

web browser A web browser, often shortened to browser, is an application for accessing websites. When a user requests a web page from a particular website, the browser retrieves its files from a web server and then displays the page on the user's scr ...

and related

applications Application may refer to: Mathematics and computing * Application software, computer software designed to help the user to perform specific tasks ** Application layer, an abstraction layer that specifies protocols and interface methods used in a ...

although most are

browser extension A browser extension is a software module for customizing a web browser. Browsers typically allow users to install a variety of extensions, including user interface modifications, cookie management, ad blocking, and the custom scripting and st ...

s. Mozilla provides add-ons to users via its official add-on

website A website (also written as a web site) is any web page whose content is identified by a common domain name and is published on at least one web server. Websites are typically dedicated to a particular topic or purpose, such as news, educatio ...

. In 2017, Mozilla enacted major changes to the

application programming interface An application programming interface (API) is a connection between computers or between computer programs. It is a type of software Interface (computing), interface, offering a service to other pieces of software. A document or standard that des ...

(API) for extensions in Firefox, replacing the long-standing

XPCOM Cross Platform Component Object Model (XPCOM) is a cross-platform component model from Mozilla. It is similar to Component Object Model (COM), Common Object Request Broker Architecture (CORBA) and system object model (SOM). It features multiple ...

-based add-on APIs with the WebExtensions API that is modeled after

Google Chrome Google Chrome is a web browser developed by Google. It was first released in 2008 for Microsoft Windows, built with free software components from Apple WebKit and Mozilla Firefox. Versions were later released for Linux, macOS, iOS, iPadOS, an ...

's API. Thus add-ons that remain compatible with Firefox are now largely compatible with Chrome as well. As of January 2024, there are more than 36,000 add-ons and over 495,000 themes available for Firefox.

Add-ons categories

Themes

Early versions of Firefox supported themes that could greatly change the appearance of the browser, but this was scaled back over time. Current themes are limited to changing the background and text color of toolbars, formerly called ''personas'', now called ''Firefox Themes''.

WebExtensions

Starting with Firefox 57, only the new WebExtensions API is supported for extensions; relegating the older extension technology as legacy.

Legacy extensions

Prior to 2017,

supported extensions developed via various APIs: XUL,

, and

Jetpack A jet pack, rocket belt, rocket pack or flight pack is a device worn as a backpack which uses jets to propel the wearer through the air. The concept has been present in science fiction for almost a century and the first working experimental d ...

now refers to these as legacy extensions.

Plug-ins

Plug-ins are no longer supported in Firefox. In the past, they were used to handle

media type In information and communications technology, a media type, content type or MIME type is a two-part identifier for file formats and content formats. Their purpose is comparable to filename extensions and uniform type identifiers, in that they ide ...

s for which the application did not have built-in capability. They were

deprecated Deprecation is the discouragement of use of something human-made, such as a term, feature, design, or practice. Typically something is deprecated because it is claimed to be inferior compared to other options available. Something may be deprec ...

due to security concerns and improvements in

Web API A web API is an application programming interface (API) for either a web server or a web browser. As a web development concept, it can be related to a web application's client side (including any web frameworks being used). A server-side web AP ...

s. The last one that was officially supported was

Adobe Flash Player Adobe Flash Player (known in Internet Explorer, Firefox, and Google Chrome as Shockwave Flash) is a discontinuedExcept in China, where it continues to be used, as well as Harman for enterprise users. computer program for viewing multimedia ...

, which Adobe discontinued in 2020.

Security

Mozilla had no mechanism to restrict the privileges of legacy Firefox extensions. This meant that a legacy extension could read or modify the data used by another extension or any file accessible to the user running Mozilla applications. But the current WebExtensions API imposes security restrictions. Starting with Firefox 40, Mozilla began to roll out a requirement for extension signing. It is now required in all official Firefox releases.

Website

The Mozilla add-ons website is the official repository for Firefox add-ons. In contrast to

mozdev.org mozdev.org was a website that offered free project hosting, and software development tools to the Mozilla community. Site hosted extensions for Firefox, Thunderbird and SeaMonkey and stand-alone Mozilla-based applications. It was free to set up ...

which provides free hosting for Mozilla-related projects, the add-ons site is tailored for

users Ancient Egyptian roles * User (ancient Egyptian official), an ancient Egyptian nomarch (governor) of the Eighth Dynasty * Useramen, an ancient Egyptian vizier also called "User" Other uses * User (computing), a person (or software) using an ...

. By default, Firefox automatically checks the site for updates to installed add-ons. In January 2008, Mozilla announced that the site had accumulated a total of 600 million add-on downloads and that over 100 million installed add-ons automatically check the site for updates every day. In July 2012, the total had increased to 3 billion downloads from the site.

References

External links

Official add-on website

WebExtensions API reference documentation

Extension Workshop, Mozilla's site for Firefox extension developer documentation
{{Mozilla Mozilla Free software websites