Firefox Firebug Plugin
Firebug integrates with Firefox to put a wealth of development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.
Project homepage: http://www.getfirebug.com/
Mozilla plugin page: https://addons.mozilla.org/en-US/firefox/addon/1843?id=1843&application=firefox