Dark Reading has a great roundup of "hacker" Firefox extensions, including ones that let you modify your cookies and user-agent string, examine and manipulate web-pages in detail, and debug (and spot vulnerabilities in) Javascript/AJAX.
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 bar
(via Schneier on Security)
