Jetpack
https://jetpack.mozillalabs.com/
Brought to you by Mozilla Labs. Required knowledge of HTML, CSS and maybe some JavaScript. You don’t have to know XUL or XML. Mozilla Labs says on its website: “Jetpack is a newly formed experiment in using open Web technologies to enhance the browser, with the goal of allowing anyone who can build a Web site to participate in making the Web a better place to work, communicate and play. In short, Jetpack is an API for allowing you to write Firefox add-ons using the web technologies you already know”
If you always wanted to create your own Firefox Add-On, now may be your chance. Have fun! =)