http://www.mozilla.org/projects/venkman/
Venkman is the code name for Mozilla's JavaScript Debugger. Venkman aims to provide a powerful JavaScript debugging environment for Mozilla based browsers, including Firefox 2.x, the Netscape 7.x series of browsers and Mozilla Seamonkey 1.x. It does not include Gecko-based browsers such as K-Meleon 1.02, Galeon 2.x and Netscape 8.x. The debugger is available as an add-on package in XPI format and has been provided as part of the Mozilla install distribution since October 3rd 2001.