Are you responsible for, or involved in the development of a Firefox feature? Would you like to make that feature available to add-on developers? If so, then this is the session for you! During this session you will learn what WebExtensions APIs are, why they are important to Firefox's future, and how (at a high level) to write them. We'll cover two paths available to you for writing WebExtension APIs, describe some APIs that have already been implemented, and point you at the resources you'll need to write an API. You will leave the session knowing whether writing a WebExtension API is for you, and if so what steps to take to make it happen. This target audience for this session is developers and product managers of Firefox features, although anyone involved in the development of Firefox could benefit.