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:
Also, two great videos to help you get started are below:
Extension Development 101
Extensions Bootcamp: Zero to “Hello World” in 45 Minutes
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!