Browser
plug-ins are a different type of module and no longer supported by the major browsers.[2][3] One difference is that extensions are distributed as
source code, while plug-ins are
executables (i.e.
object code).[2] The most popular browser,
Google Chrome,[4] has over 100,000 extensions available[5] but stopped supporting plug-ins in 2020.[6]
History
Internet Explorer was the first major browser to support extensions, with the release of
version 4 in 1999.[7]Firefox has supported extensions since its launch in 2004.
Opera and Chrome began supporting extensions in 2009,[8] and
Safari did so the following year.
Microsoft Edge added extension support in 2016.[9]
API conformity
In 2015, a community group formed under the
W3C to create a single standard
application programming interface (API) for browser extensions.[10] While this particular work did not reach fruition,[11] every major browser now has the same or very similar API due to the popularity of
Google Chrome.[4]
Chrome was the first browser with an extension API based solely on
HTML,
CSS, and
JavaScript. Beta testing for this capability began in 2009,[12][13] 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.[14] In the same year, Chrome overtook
Internet Explorer as the world's most popular browser,[15] and its
usage share reached 60% in 2018.[16]
Because of Chrome's success,
Microsoft created a very similar extension API for its
Edge browser, with the goal of making it easy for Chrome extension developers to port their work to Edge.[17] But after three years Edge still had a disappointingly small market share, so Microsoft rebuilt it as a
Chromium-based browser.[18][19] (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.[20]
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.[21] This change was enacted in 2017.[22][23] Firefox extensions are now largely compatible with their Chrome counterparts.[24]
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.[25] Extensions were later enabled in the
iOS version for the first time.[26]
In 2021, these browser vendors formed a new W3C community group, called WebExtensions, to "specify a model, permissions, and a common core of APIs".[27] However, Google joined this during its overhaul of Chrome's extension API, known as
Manifest V3, which greatly reduces the capability of
ad blockers and
privacy-related extensions.[28][29][30] Thus the WebExtensions group is viewed by some extension developers as nothing more than Google imposing its Manifest V3 design.[31][32][33]
Unwanted behavior
Browser extensions typically have access to sensitive data, such as
browsing history, and they have the ability to alter some browser settings, add user interface items, or replace website content.[34][35] As a result, there have been instances of
malware, so users need to be cautious about what extensions they install.[36][37][38][39]
Some
Google Chrome extension developers have sold their extensions to third-parties who then incorporated
adware.[41][42] In 2014,
Google removed two such extensions from the
Chrome Web Store after many users complained about unwanted pop-up ads.[43] The following year, Google acknowledged that about five percent of visits to its own websites had been altered by extensions with adware.[44][45][46]
^Stuwe, Cuyler (29 December 2021).
"Re: Do not outlaw dynamic code". GitHub. Nobody outside of Google really has any meaningful sway, since Chrome currently owns nearly all of the market share; Google sets the rules, other people nitpick minor details, and everyone pretends that everyone had a meaningful say.
^"Re: Do not outlaw dynamic code". GitHub. 2 January 2022. Google has shown no interest whatsoever in deviating from their original plans. So, it's quite clear what's happening here. MV3 is a business decision, not an engineering decision.