As of July 2017, we are moving away from calling extensions built with WebExtensions APIs "WebExtensions", and towards simply calling them "extensions". "WebExtensions" will still be used to describe the API that extensions are built on.
When we first decided to move Firefox extensions to a new API, we needed a name for it that 1) was searchable 2) was distinguishable from legacy technology and 3) signals its cross-browser compatibility. “WebExtensions” did all three, and we used it to describe extensions built with this new API.
Now that we’re close to completing the transition and supporting just one API, we need to gradually drop the term “WebExtensions” when describing extensions built using the WebExtensions API.
- Add-ons: an umbrella term that describes different types of code that modify Firefox. Mostly used on AMO. Includes extensions, themes, plug-ins, dictionaries, etc.
- Extensions: pieces of code that extend the capabilities of a browser. In Firefox, extensions are a type of add-on.
- Browser extensions: cross-browser extensions implemented using the Browser Extensions standard. (This article illustrates it nicely).
- WebExtensions: as a standalone noun, this is a deprecated term that used to describe an extension built using WebExtensions APIs. With WebExtensions APIs becoming the only supported API in Firefox 57, we are simply calling them "extensions." See below on how "WebExtensions" can still be used as a standalone adjective.
- Legacy extensions: Firefox extensions built using legacy APIs or the Add-on SDK.
- Avoid using "add-ons" and "extensions" interchangeably - The terms “add-ons” and “extensions” should be used in accordance with their definition and the context in which they appear.
- Add-ons is a Firefox-specific term. There are different kinds of add-ons that you can use to customize Firefox, including extensions and themes. Use "add-ons" when talking about the customization of Firefox in general.
- Extensions add new features and functionality to browsers. Use "extensions" when talking about the type of add-on that adds new features and functionality to Firefox, or when talking about the customization of browsers in general.
- "WebExtensions" refers to the technology, and not extensions built with the technology - For example, “Cool new extensions” rather than “Cool new WebExtensions”
- "WebExtensions" can still be used as a standalone term - It is ok to use the word "WebExtension(s)" without "API" following it, as long as you are describing the technology. For example:
- To distinguish from a legacy extension: "Your WebExtension add-on displays a 'compatible with Firefox 57+' banner."
- To describe the new system: "We're moving to a WebExtensions model for add-on development."
- "API" or "APIs"? - The system, or the technology underlying Firefox extension development, is the WebExtensions API. Comprising it are WebExtensions APIs that expose certain parts of Firefox to developers.
- "Browser extensions" - For MDN extension documentation, formal cases (like the main title of the page) should say "Browser extensions", and thereafter "extensions" as an informal shorthand
The site where you go to list and download add-ons is "addons.mozilla.org (AMO)" on the first mention, and "AMO" after that.
When in doubt, please ask :kev, :jorgev, :wbamberg, or :amyt on irc.mozilla.org