WebExtensions are a way to write Firefox extensions that are compatible with other browsers such as Google Chrome and Opera. Microsoft plans to bring support to their Edge browser soon. Questions requiring a MCVE (i.e. debugging questions) should include your manifest.json file in addition to all other files needed to duplicate the problem.
WebExtensions are a new way to write Firefox extensions. It is available in all current versions of Firefox (version 48.0+).
The WebExtensions API is designed for cross-browser compatibility. To a large extent, the API is compatible with the extension API supported by Google Chrome and Opera, with Microsoft Edge following soon.
The documentation for Firefox WebExtensions can be found at MDN :: Add-ons - WebExtensions.
The WebExtensions API is under active development. What is supported improves in each newer version of Firefox. You should pay attention to the Browser Compatibility section on the API documentation page for any part of the API you are using to verify the portions of the API you are using are supported in the version of Firefox you are using. You may want to strongly consider testing your extension with either Firefox Developer Edition, or Firefox Nightly.
Minimal, Complete, and Verifiable Examples will almost always need a manifest.json file in order to be complete, in addition to any other files needed. The manifest.json file contains key information which is necessary in many situations to determine why an extension is not working.
For more information on Firefox Add-on types see the full documentation on Mozilla Developer Network: Add-ons.
