Add-on is the

Mozilla Mozilla (stylized as moz://a) is a free software community founded in 1998 by members of Netscape. The Mozilla community uses, develops, spreads and supports Mozilla products, thereby promoting exclusively free software and open standards, w ...

term for

software Software is a set of computer programs and associated software documentation, documentation and data (computing), data. This is in contrast to Computer hardware, hardware, from which the system is built and which actually performs the work. ...

modules that can be added to 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 current ...

web browser A web browser is application software 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 screen. Browsers are used o ...

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 c ...

. Mozilla hosts them on its official add-on

website A website (also written as a web site) is a collection of web pages and related content that is identified by a common domain name and published on at least one web server. Examples of notable websites are Google, Facebook, Amazon, and Wi ...

. Browser extensions are the primary type of add-on. In 2017, Mozilla enacted major changes to the application programming interface (API) for extensions in Firefox, replacing the long-standing

XUL XUL ( ), which stands for XML User Interface Language, is a user interface markup language developed by Mozilla. XUL is an XML dialect for writing graphical user interfaces, enabling developers to write user interface elements in a manner sim ...

and

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

APIs with the

WebExtensions API Mozilla Firefox has features that allow it to be distinguished from other web browsers, such as Chrome and Internet Explorer. Major differences To avoid interface bloat, ship a relatively smaller core customizable to meet individual users' ne ...

that is modeled after Google Chrome's API. Thus add-ons that remain compatible with Firefox are now largely compatible with Chrome as well.

Current add-ons

Extensions

WebExtensions

Starting with Firefox 57, only the new

is supported.

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. (These lightweight themes were formerly called personas.)

Historical add-ons

Extensions

Legacy extensions

Prior to 2017,

supported

extensions Extension, extend or extended may refer to: Mathematics Logic or set theory * Axiom of extensionality * Extensible cardinal * Extension (model theory) * Extension (predicate logic), the set of tuples of values that satisfy the predicate * E ...

developed with different APIs:

, and

Jetpack A jet pack, rocket belt, or rocket pack is a device worn on the back which uses jets of gas or liquid to propel the wearer through the air. The concept has been present in science fiction for almost a century and became widespread in the 1960s. ...

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

deprecated In several fields, especially computing, deprecation is the discouragement of use of some terminology, feature, design, or practice, typically because it has been superseded or is no longer considered efficient or safe, without completely removing ...

due to security concerns and improvements in

Web API A web API is an application programming interface for either a web server or a web browser. It is a web development concept, usually limited to a web application's client-side (including any web frameworks being used), and thus usually does not ...

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 computer software for viewing multimedia contents, executing rich Internet applications, and streaming audio and video content created on the ...

, which Adobe discontinued in 2020.

Restrictions

had no mechanism to restrict the privileges of legacy

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 many 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

add-ons website is the official repository for

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 a ...

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 user is a person who ...

. 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