If you are getting into website development, firebug is a key component in your tool box. It allows you to scope through the code (mainly HTML and CSS) of any site on the web. It also allows you to make visual changes on the front end, without having to save your Style sheets, over and over and over again.
Firebug is a must have for all web developers or managers.
Here’s a great reference video for using firebug from the guys at Woo Themes.