Firebug is a free and open-source web browser extension for Mozilla Firefox that facilitates the live debugging, editing, and monitoring of any website's CSS, HTML, DOM, XHR, and JavaScript. It is a free and open source Javascript debugging application that's reliable and powerful.
Here are some screenshots to illustrate the features and user interface of this application:
Noscript for Firefox
anonymoX
FireFTP
Firemin
HTTPS Everywhere