This is a video of the creator of Firebug showing more advanced ways to use it. If your using Firebug it's a must watch.
http://ajaxian.com/archives/joe-hewitt-firebug-power-user-demo
There is a very nice article here on using the Firefox Firebug extension to debug Ajax and Javascript.
http://www.ddj.com/dept/debug/196802787
I especially like the new profiling functionality
