Add-ons Builder lets you quickly and easily develops an extension for Firefox using common web technologies. It is an online coding tool that you can do with the help of a few extra ready-made elements.
And if you don’t want to start from scratch, you can use a variety of APIs in the core library to add functionality to Firefox, including widget and context-menu. Check out other add-ons for inspiration. And take advantage of third-party libraries with additional APIs.
There is also a short tutorial to help you build an extension or add-on. More information available at Mozilla.