Firebug is a plugin for the Mozilla Firefox web browser that allows you to develop and modify web sites from directly inside Firefox.
This could be a great tool due to the fact that it is so integrated with the web browser. I usually use Dreamweaver at work. This is a web development environment (a very expensive one!) that gives you the ability to preview your web page as you build it.
Dreamweaver renders the web page as close as possible to what it would actually look like on a web browser. Of course, there may be discrepancies between web browsers, but this is a close approximation of what the page would look like.
Firebug will allow web site designers to quickly see how changes to a web site’s code will be reflected in Firefox. This is a great tool for web developers worldwide. Best of all, it is free!
You can read a good review of some cool features found in Firebug in an article on Eric’s Weblog. He also has cool pictures to show what the interface is like.