Firebug for Firefox
Javascript debugging, DOM inspectors, javascript command line, and an AJAX log are just a few of the great features of Firebug.
FireBug lets you explore the far corners of the DOM by keyboard or mouse. All of the tools you need to poke, prod, and monitor your JavaScript, CSS, HTML and Ajax are brought together into one seamless experience, including a debugger, an error console, command line, and a variety of fun inspectors.
Brought to you by one of the creators of Firefox itself, Joe Hewitt. You can download it at his site or at Mozilla.org
