Firebug



The Firebug extension for Mozilla Firefox allows the debugging, editing, and monitoring of any website's CSS, HTML, DOM, 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 execute scripts and load images on the page.

You can change the entire layout of the website by modifying the existing source code to see a live effect on the screen. This does not change the site permanently, it only gives you a visual reference to what it would look like if it were changed that way.

Firebug is free and open source, licensed under the BSD license. Firebug was initially written by Joe Hewitt, one of the original Firefox creators. The Firebug Working Group oversees the open source development and extensions of Firebug. Approximately 1.7 million users have Firebug installed as of Oct. 2009.

In addition to debugging web sites, Firebug is the most popular and powerful web development tool for web security testing and for web site performance analysis.

It is possible to add a Firebug button to one of your toolbars, and also to your status bar.