Today I downloaded the FireBug Add-on for FireFox. It is very cool tool. I thought the IE toolbar is very useful tool for debugging the UI related issue on the web app. So far, I only play a few functioanlities and I am very impressed. Specially, the "inspect" function. Once you click the button and move the mouse over the page you are looking at, the FireBug will take you directly to the section over the rendered HTML code and show all the style information on the right panel.
You can get FireBug from http://getfirebug.com/