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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, HTTP cookie management, ad blocking, and the custom userscript and styling of .
Browser plug-ins are a different type of module and no longer supported by the major browsers. One difference is that extensions are distributed as source code, while plug-ins are (i.e. object code). The most popular browser, Google Chrome, has over 200,000 extensions available but stopped supporting plug-ins in 2020.
Chrome was the first browser with an extension API based solely on HTML, CSS, and JavaScript. Beta testing for this capability began in 2009, and the following year Google opened the Chrome Web Store. As of June 2012, there were 750 million total installations of extensions and other content hosted on the store. In the same year, Chrome overtook Internet Explorer as the world's most popular browser, and its usage share reached 60% in 2018.
Because of Chrome's success, Microsoft created a very similar extension API for its Microsoft Edge browser, with the goal of making it easy for Chrome extension developers to port their work to Edge. But after three years Edge still had a disappointingly small market share, so Microsoft rebuilt it as a Chromium-based browser. (Chromium is Google's open-source project that serves as the functional core of Chrome and many other browsers.) Now that Edge has the same API as Chrome, extensions can be installed directly from the Chrome Web Store.
In 2015, Mozilla announced that the long-standing XUL and XPCOM extension capabilities of Firefox would be replaced with a less-permissive API very similar to Chrome's. This change was enacted in 2017. Firefox extensions are now largely compatible with their Chrome counterparts.
Apple was the last major exception to this trend, but support for extensions conforming to the Chrome API was added to Safari for macOS in 2020. Extensions were later enabled in the iOS version for the first time.
In 2021, these browser vendors formed a new W3C community group, called WebExtensions, to "specify a model, permissions, and a common core of APIs". However, Google joined this during its overhaul of Chrome's extension API, known as Manifest V3, which greatly reduces the capability of ad blocking and Internet privacy-related extensions. Thus the WebExtensions group is viewed by some extension developers as nothing more than Google imposing its Manifest V3 design.
There have also been cases of applications installing browser extensions without the user's knowledge, making it hard for the user to uninstall the unwanted extension.
Some Google Chrome extension developers have sold their extensions to third-parties who then incorporated adware. In 2014, Google removed two such extensions from the Chrome Web Store after many users complained about unwanted pop-up ads. The following year, Google acknowledged that about five percent of visits to its own websites had been altered by extensions with adware.
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