The Web Developer extension is another must-have. This extension has too many features to list, and I typically find a new one each time I use it. Some of the features include editing practically every aspect of the page (HTML, CSS, cookies), viewing all elements in the page in a sort of WYSIWIG way, and converting form submission methods from POSTs to GETs and vice versa. I use it primarily for dissecting Web pages, but it comes in handy to convert the POSTs to GETs in order to easily manipulate the values in the URL address barLink (via Schneier on Security)
I write books. My latest is a YA science fiction novel called Homeland (it's the sequel to Little Brother). More books: Rapture of the Nerds (a novel, with Charlie Stross); With a Little Help (short stories); and The Great Big Beautiful Tomorrow (novella and nonfic). I speak all over the place and I tweet and tumble, too.