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:

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