If you’re never built an add-on but really want to enter the Extend Firefox 3.5 contest, we have some resources that will help get you up to speed. First we have Robert Nyman’s excellent tutorial on building add-ons:
How to develop a Firefox extension by Robert Nyman
Also, two great videos to help you get started are below:
Extension Development 101
Extension Development 101 from rhian on Vimeo.
Extensions Bootcamp: Zero to “Hello World” in 45 Minutes
Extension Bootcamp (Mozilla Labs Design Challenge: Spring 09) from Mozilla Labs – Concept Series on Vimeo.
And don’t forget that we’ve created an Extend Firefox 3.5 discussion group to answer an questions you have about the contest. Good luck!