There was a buzz around Firebug a couple of months ago. A lot of developers installed it and enjoy every second they use it. There are some others that missed the train and are still using alerts to debug. If you're from that second category, here's your chance to get back on tracks.
If you have an error, just click on it and Firebug will open the source code and highlight it.
Every ajax request is also shown here along with its parameters and the response from the server.
The debugger is insanely efficient, it offers everything you would expect from a debugger : breakpoints, step-by-step, watch expressions and functions stack. Do you need anything more?