Changes

The MDN web docs site has an [https://developer.mozilla.org/en-US/Add-ons/WebExtensions entire section ] dedicated to extension development with the WebExtensions]API. It provides a quick overview to get you started as well as complete reference documentation for the entire WebExtensions API.

The WebExtensions [https://developer.mozilla.org/Add-ons/Distribution Publishing and Distributing] section of MDN provides a complete guide to getting your extension approved and published.

Yes. [https://webextensions-experiments.readthedocs.io WebExtension Experiments] allow you to expose and test almost any feature as a new WebExtensions API without having to build Firefox. It can help you understand how an API might work and what its limitations might be. Prototype WebExtension experiments can be run in the Firefox [https://www.mozilla.org/firefox/channel/desktop/ Nightly] build. Keep in mind, though, that the existence of an experiment doesn’t guarantee that the feature will ultimately become an official WebExtension API.