Firebug is a discontinued free and open-source[4] web browser extension for Mozilla Firefox[2] that facilitated the live debugging, editing, and monitoring of any website's CSS, HTML, DOM, XHR, and JavaScript.

[6] In its 0.2.3 release on January 31, 2006, it was time that F12 was used to open/close the developer tools panel in web browsers.

[12] Firebug 2.0 introduced many new features to the Firebug extension including JavaScript syntax highlighting, pretty print for minified JavaScript code, and a DOM Event Inspector to handle all event handlers on a web page.

[16] Its net panel can monitor URLs that the browser requests, such as external CSS, JavaScript, and image files.

Furthermore, users can click on any visible HTML elements on a web page to access its CSS property values.

[18] Firebug's script tab enables users to set breakpoints and step through lines of code.

[19] Additionally, Firebug can navigate directly to a line of JavaScript code, watch expressions, call stacks, and launch the debugger in the event an error occurs during execution.

Firebug provides a separate text editor to modify the JavaScript and see immediate results on the user's browser.