Add-on (Mozilla)

Add-on is the Mozilla term for software modules that can be added to the Firefox web browser and related applications. There are three types: extensions, themes, and plug-ins.[1] Mozilla hosts them on its official add-on website.[2]

In 2017, Mozilla enacted major changes to the application programming interface (API) for add-ons in Firefox.[3][4][5] The long-standing XUL and XPCOM capabilities were replaced by an API modeled after Google Chrome's; Firefox extensions are now largely compatible with their Chrome counterparts.[6] Plug-ins were deprecated, with the exception of the Adobe Flash Player.[7]

Types of add-ons


Prior to 2017, Firefox supported extensions developed with different APIs: XUL, XPCOM, and Jetpack.[8] Mozilla now refers to these as legacy extensions.[5] Starting with Firefox 57, only the new WebExtensions API is supported.[4][9]


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.[10]


Plug-ins are used to handle media types for which the application does not have built-in capability. They are third-party executables that interface via NPAPI. But plug-ins are now deprecated, due to security concerns and improvements in Web APIs.[11] The only plug-in still officially supported by Firefox is the Adobe Flash Player,[7] which Adobe will cease to update in 2020.[12]


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.[13] But the current WebExtensions API imposes many restrictions.[14]

Starting with Firefox 40, Mozilla began to roll out a requirement for extension signing.[15] It is now required in all official Firefox releases.[16]


Add-ons for Firefox
Type of site
Hosts add-ons
OwnerMozilla Foundation
RegistrationFree; only needed for developers or for special features
Current statusActive

The Mozilla add-ons website is the official repository for Firefox add-ons.[2] In contrast to which provides free hosting for Mozilla-related projects, the add-ons site is tailored for users. By default, Firefox automatically checks the site for updates to installed add-ons.[17]

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.[18] In July 2012, the total had increased to 3 billion downloads from the site.[19]


  1. "Add-ons". Mozilla Developer Network. Retrieved 2016-01-21.
  2. "Official Add-on website". Mozilla. Retrieved 10 February 2019.
  3. "The Future of Developing Firefox Add-ons". Mozilla Add-ons Blog. Retrieved 2018-12-15.
  4. "Upcoming Changes in Compatibility Features". Mozilla Add-ons Blog. Retrieved 2018-12-15.
  5. "How to enable legacy extensions in Firefox 57 - gHacks Tech News". Retrieved 2018-12-14.
  6. "Porting a Google Chrome extension". Mozilla. Retrieved 30 December 2018.
  7. "Why do Java, Silverlight, Adobe Acrobat and other plugins no longer work?". 2017-01-30. Archived from the original on 2017-03-07. Retrieved 2017-03-17.
  8. Chapter 2: Technologies used in developing extensions - Firefox addons developer guide | MDN. Retrieved on 2013-07-21.
  9. "Browser extensions". MDN. Mozilla. Retrieved 10 February 2019.
  10. "Themes for Firefox".
  11. Smedberg, Benjamin (8 October 2015). "NPAPI Plugins in Firefox". Future Releases. Mozilla Foundation.
  12. Lardinois, Frederic. "Get ready to finally say goodbye to Flash — in 2020". TechCrunch. Retrieved 2017-07-25.
  13. "Abusing, Exploiting and Pwning with Firefox Add-ons" (PDF). Archived from the original (PDF) on 2016-03-05. Retrieved 2013-10-23.
  14. "Security Best Practices". MDN. Mozilla. Retrieved 10 February 2019.
  15. Fisher, Dennis. "Firefox 40 Begins Warning Users About Unsigned Add-Ons". Threatpost. Retrieved 20 August 2015.
  16. "Extension Signing". Wiki. Mozilla. Retrieved 20 August 2015.
  17. "Updates". Mozilla Developer Network. Retrieved 2017-10-28.
  18. Scott, Justin (30 January 2008). "600,000,000 Add-on Downloads". Blog of Metrics. Mozilla Foundation.
  19. "Firefox Add-ons Cross More Than 3 Billion Downloads!". The Mozilla blog. Retrieved 1 November 2013.
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