Mozilla’s Add-on Builder is an online coding tool that helps you build your own extension or add-on for Firefox using common web technologies.
The Add-on Builder from Mozilla makes it easy to write, test, and build add-ons with the help of a few extra ready-made elements. Additionally, a step-by-step guide is also there to help you.
Moreover, if you don’t have an idea to build an add-on, you can check out other add-ons for inspiration, take advantage of third-party libraries and you can use a variety of APIs in the core library to add functionality to Firefox, including widget and context-menu. [via]