Browser Addons

Mozilla/Firefox

• The Firebug is the debugger of choice for Dojo and Firefox. Consult Cheap Debugging for details.
• JavaScript Debugger, a/k/a Venkman has been around for years, and has adherents. Venkman seems to be dormant, gaining no new features but being ported verbatim to new versions of Firefox.
• Live HTTP Headers is useful for debugging HTTP traffic . Although Firebug is good for analyzing HTTP traffic within the page, as in XHR, Live HTTP Headers excels at between-page HTTP requests.
• The Web Developer Toolbar is a favorite of Web Designers, and includes rulers and positioning features that Firebug does not.

Internet Explorer

• IE Web Developer is a commercial product with Firebug-like features such as JavaScript debugging.
• IE Developer Toolbar, a free download from Microsoft, is useful for page design.
• The Microsoft Script Editor integrates with Internet Explorer and is available as part of office 2003. It is not included in the default install but you can get it from the installation cd's.
• Web Developer Helper - HTTP logging, script debugging and a DOM inspector.

Safari

Drosera is the "native" JavaScript debugger for Safari, and is written by the folks from Webkit, on which Safari is based. The Safari Developer FAQ has some general information about developing with Safari, as well as instructions on how to turn on a debug menu that allows showing a JavaScript console.