I had always heard that the firefox addon Firebug was very useful...and it was great when selecting objects or finding errors. But it was when I started using the Copy Css Path right-click that started opening my eyes. Makes mining down to the targeted css overide code so easy.
Also selecting the item now brings new layout window on the right hand side where you can simply change attributes and see their effects immediately on the browser, and btw, not it is available for chrome as well.