Venkman is a Javascript Debugger as a FireFox extenstion.It's at least powerful than IE's default script debugger(not Visual InterDev's).You can watch varaiable,set breakpoint and use "step over" "step into" "step out" "continue" buttons to debug your niffy javascript codes.
It's ease to use.And tutorial is HERE:http://www.svendtofte.com/code/learning_venkman/index.php

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