If you do web design or web application development check out the
Web Developer Extension for Firefox. An incredibly handy toolbar addition to Firefox, It includes handy utilities like the ability to edit CSS in real time, on any site you are browsing. You can even edit the HTML source of web pages directly and see real-time updates. That's not all however, it includes more options to do everything from visualize page elements, resize the browser window to test content, validate page code with a click, autopopulate form fields when building web applications, and more.
