Furebug - great web development tool

This is an excellent add-on for the FireFox. The Furebug lets debug front-end stuff with pleasure because it provides such cool features as:

• JavaScript debugger
• Nice watch panel that shows all about JavaScript objects, functions, variables
• DOM Viewer
• CSS Viewer
• Dynamic HTML Viewer that can higliht elements on the FF when your mouse is over HTML tag
• Inspector - to select any object on the rendered page and show it's HTML
• Console
• Editor
• Features to speed up your code

Very useful tutorial is here. Also you can find some videos about Firebug.