Must-haves

• “Venkman aims to provide a powerful Java Script debugging environment for Mozilla based browsers.”. Venkman provides Java Script debugging, including breakpoints and profiling. See http://www.mozilla.org/projects/venkman/
• “The Web Developer extension for Mozilla Firefox and Mozilla adds a menu and a toolbar to the browser with various web developer tools.” See http://chrispederick.com/work/firefox/webdeveloper/
• “The Internet Explorer Developer Toolbar provides several features for exploring and understanding Web pages.” See
  http://www.microsoft.com/downloads/details.aspx?FamilyID=E59C3964-672D-4511-BB3E-2D5E1DB91038&displaylang=en
• The Java Script Console (a bookmarklet which allows you to test bits of javascript on-the-fly)
  http://www.squarefree.com/bookmarklets/webdevel.html
• Firebug is a great extension to track http requests, inspect elements and keep a track of javascript errors. It’s available here: https://addons.mozilla.org/firefox/1843/ and is highly recommended by Mozilla developers themselves.

Doesn’t hurt either

• “tamperdata is an extension to track and modify http/https requests.”. Great for seeing what does AJAX requests do. See http://tamperdata.mozdev.org/
• Internet Explorer (for Windows): get the Microsoft Script Debugger
• On Safari, activate the debug menu. See http://developer.apple.com/internet/safari/faq.html#anchor14
• An alternative toolbar for Internet Explorer can be found here provided by The Web Accessibility Tools Consortium [WAT-C]
• Sahi is a javascript based lightweight functional testing tool. Use it to test drag-drop and other aspects of your UI. It’s available here: http://sahi.sourceforge.net/