The official launch of WebExtensions is happening in Firefox 48, but much of what you need is already supported in Firefox and AMO (addons.mozilla.org). The best place to get started with WebExtensions is MDN, where you can find a trove of helpful information. I’d like to highlight a couple of recent additions that you might find useful:

• Comparison with the Add-on SDK chart – SDK developers can see how different SDK APIs and techniques map to their equivalents in the WebExtensions world, so it’s easier to migrate. It’s not a one-to-one match, but it comes close at least for high-level APIs.
• Comparison with XUL/XPCOM extensions chart – Similar to the SDK comparison chart, but a bit trickier to do, given the breadth of the XPCOM platform. We’re using the commonly-used components exposed in Services.jsm for the API mapping table. If you see anything missing on that table, please edit it yourself or let us know.
• How-to guides – There’s a new How to section on the WebExtensions home page that guides you through some common add-on developer cases, like intercepting web requests and adding a button to the toolbar. Let us know if you have other requests for these how-to guides by commenting below, or joining the dev-addons mailing list.

Thank you to Will Bamberg for doing the bulk of this work. Remember that MDN is a community wiki, so anyone can help!

https://blog.mozilla.org/addons/2016/07/21/new-webextensions-guides-and-how-tos-on-mdn/