Firebug

The Firebug extension for Mozilla Firefox allows the debugging, editing, and monitoring of any website’s CSS, HTML, DOM, and JavaScript, and provides other Web development tools. It also has a JavaScript console for logging errors and watching values, as well as a “Net” feature which monitors the amount of time in milliseconds it takes to load and execute scripts and images on the page. Firebug’s official website has a complete guide to what it can do. Firebug is free and open source, licensed under the BSD license.