Fernando Meyer

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.A quick overview of FireBug’s features:

• JavaScript debugger for stepping through code one line at a time
• Status bar icon shows you when there is an error in a web page
• A console that shows errors from JavaScript and CSS
• Log messages from JavaScript in your web page to the console (bye bye “alert debugging”)
• An JavaScript command line (no more “javascript:” in the URL bar)
• Spy on XMLHttpRequest traffic
• Inspect HTML source, computed style, events, layout and the DOM

https://addons.mozilla.org/firefox/1843/