Add WebExtension topic

topics/web-extension/index.md

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+---
+display_name: Web extension/browser extension
+short_description: A browser extension is a way to add customization to a browser.
+topic: web-extension
+aliases: browser-extension, webextension
+related: chrome-extension, firefox-extension, safari-extension
+url: https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions
+wikipedia_url: https://en.wikipedia.org/wiki/Browser_extension
+---
+A **browser extension** or **web extension** (also written as **WebExtension**) is a way to customize various aspect of a browser's functionality:
+* user interface modifications
+* cookie management
+* ad blocking
+* custom scripting and styling of web pages
+
+Other than browser plugins, they are distributed as source code, while plugins are executable.
+
+While there is a [common standard](https://en.wikipedia.org/wiki/Manifest_file#WebExtension_manifest) (to which Mozilla switched in 2015), there are slight differences between the browser's implementations of said standard:
+* [Mozilla Firefox](https://github.com/topics/firefox-extension) (gecko engine)
+    * gecko uses `browser` when referring to the browser object in scripts, but supports using `chrome`
+    * supports non-persistent background scripts called "event-driven background scripts"
+* [Google Chrome](https://github.com/topics/firefox-extension) (webkit engine)
+    * webkit uses `chrome` when referring to the browser object in scripts
+    * instead of event-driven background scripts, Chrome has service workers
+* Apple's [Safari](https://github.com/topics/firefox-extension) (also webkit)
+* _Browser-specific settings can be applied to the `browser_specific_settings` key in the `manifest.json`_
+
+The WebExtension manifest standard is currently in version 3 (called "Manifest V3"), but Firefox still supports version 2 (called "Manifest V2").
+* Some key names changed (e.g. `browser_action` to `action`)
+
+An extension is roughly divided into parts, of which not all might be used:
+* **content scripts:** Scripts that manipulate the web page of the current tab
+* **background scripts:** Scripts that don't have access to the web page but can do other tasks, including displaying a popup when clicked on the extension icon
+