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A browser extension is a computer program that extends the functionality of a web browser in some way. Depending on the browser and the version, the term may be distinct from similar terms such as plug-in or add-on.
Mozilla Firefox was designed with the idea of being a small and simple web browser, delegating more advanced functions to Mozilla extensions. Microsoft Internet Explorer started supporting extensions from version 5. Google Chrome started supporting extensions from version 4. The Opera desktop web browser supported extensions from version 10. The Apple Safari web browser started supporting native extensions from version 5. The syntax for extensions may be quite different from browser to browser, or at least enough different that an extension working on a browser does not work on another one. As for search engine tools, an attempt to bypass this problem is the multitag strategy proposed by the project Mycroft, a database of search engine addons working on different browsers.
A browser toolbar is a toolbar that resides within a browser's window. All major web browsers provide support to browser toolbar development as a way to extend the browser's GUI and functionality. Browser toolbars are considered to be a particular kind of browser extensions that present a toolbar. Browser toolbars are specific to each browser, which means that a toolbar working on a browser does not work on another one.
Plug-ins add specific abilities into browsers using application programming interfaces (APIs) allowing third parties to create plug-ins that interact with the browser. The original API was NPAPI, but subsequently Google introduced the PPAPI interface in Chrome.
Browser extension development is the actual creation of an extension for a specific browser. Each browser type has its own architecture and APIs to build the extensions which requires different code and skills for each extension. Nowadays there are development frameworks which allows developers to build cross-browser extensions with only one code base and one API, eliminating the need to develop a different extension version for each one of the Browsers. Examples of those frameworks are Add-ons Framework which allows developer to build cross browser extensions for Internet Explorer, Firefox, Chrome, Safari and Opera, and the Crossrider development framework which allows developers to build cross browser extensions for Internet Explorer, Firefox, Chrome and Safari.
See also 
- Mozilla Add-ons
- Google Chrome Extensions
- List of Firefox extensions
- List of Internet Explorer add-ons
- Category:Firefox add-ons
- Category:Google Chrome extensions
- Category:Internet Explorer add-ons
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