Firebug

Firebug is one of the most popular tools used by Web developers. It’s an extension of Mozilla Firefox that facilitates editing, debugging and monitoring CSS, HTML and JavaScript on a live Web page. Firebug was developed by Joe Hewitt in 2006. Its key features are: Inspects and edits HTML Tweaks CSS to perfection Visualises CSS metrics Monitors network activity Debugs and profiles JavaScript Quickly finds errors Explores the DOM