Firebug
Web browser extension
Firebug is a discontinued free and open-source web browser extension for Mozilla Firefox that facilitated the live debugging, editing, and monitoring of any website's CSS, HTML, DOM, XHR, and JavaScript. Wikipedia
Developer(s): Firebug Working Group
License: New BSD License
Original author(s): Joe Hewitt
Stable release: 2.0.19 / February 16, 2017; 7 years ago