FireFox Addon: Firebug
Firebug Web Development
I am a big fan of Firebug. Firebug is a great learning tool. It has allowed me to take a peak a behind the scenes sorta speak. It allows you to view and edit HTML code, and much more. If you are beginning to learn web development, building your first real estate html website or tweaking a WordPress blog this is the tool for you. Firebug integration with Firefox is seamless.