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. Mozilla hosts them on its official add-on website.
In 2017, Mozilla enacted major changes to the application programming interface (API) for extensions in Firefox, replacing the long-standing XUL and XPCOM APIs with the WebExtensions API that is modeled after Google Chrome's API. Thus Firefox extensions are now largely compatible with their Chrome counterparts. At the same time, plug-ins were deprecated, with the exception of the Adobe Flash Player. (However, applications built with the UXP fork of Firefox retain the legacy add-on capabilities.)
Types of add-ons
Prior to 2017, Firefox supported extensions developed with different APIs: XUL, XPCOM, and Jetpack. Mozilla now refers to these as legacy extensions. Starting with Firefox 57, only the new WebExtensions API is supported.
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.)
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. The only plug-in still officially supported by Firefox is the Adobe Flash Player, which Adobe will cease to update in 2020.
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 many restrictions.
Type of site
|Registration||Free; only needed for developers or for special features|
The Mozilla add-ons website is the official repository for Firefox add-ons. In contrast to mozdev.org 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.
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.
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