JavaScript Debug Recommended Tools
Firefox
Firebug
Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page
https://addons.mozilla.org/firefox/addon/1843
IE
Companion.JS
Companion.JS (pronounced Companion dot JS or CJS) is a Javascript debugger for IE. The current version is 0.3.
Companion.JS adds the following features to IE :
* Detailled javascript error reporting (call stack and real file name where the error occured).
* "Firebug"-like Console API feature.
* Javascript console feature useful to inspect javascript objects at runtime.
* A toolbar icon to open the Companion.JS panel.
http://www.my-debugbar.com