Extensions for Firefox are built using WebExtensions APIs, a cross-browser system for developing extensions. To a large extent the API is compatible with the extension API supported by Google Chrome and Opera. Extensions written for these browsers will in most cases run in Firefox or Microsoft Edge with just a few changes. The API is also fully compatible with multiprocess Firefox.
In the past, you could develop Firefox extensions using one of three different systems: XUL/XPCOM overlays, bootstrapped extensions, or the Add-on SDK. By the end of November 2017, WebExtensions APIs will be the only way to develop Firefox extension, and the other systems will be deprecated.