FireBug is an amazing web development plugin for FireFox.  Like Web Developer Plugin, it lets you tweak HTML, CSS, and JavaScript on the fly.  However, it also bakes in features like visualizations to make sense of CSS and deeply nested elements easily.  It also gives you a nice drill-down report of network traffic, so you can see exactly how much each element contributes to the total page size, which is great when you're trying to trim those extra Kb off your pages.  The UI is very well thought out and easy to understand. 

Hang on to Web Developer Plugin for the validation features, but this is definitely a must have for your web development toolbox.

http://www.getfirebug.com/