There are very well known Javascript debugging tools for firefox. Those are fiebug and webdev. In fact, without those tools, javascript development would be *alert()* debugging after all.
I used MS visual studio for past 5 years and it had a great Javascript debugging tool for IE. (Last I remember, MS visual studio 2005 had a Javascript debugging tool, which can be used for IE as well as firefox.) Somehow I happened to go to this page today. This page shows three ways to debug Javascript on IE.