Cool Fire Fox Addons:

 

Web Developer

The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools

https://addons.mozilla.org/en-US/firefox/addon/60

 

Firebug

• Inspect HTML and modify style and layout in real-time
• Use the most advanced JavaScript debugger available for any browser
• Accurately analyze network usage and performance
• Extend Firebug and add features to make Firebug even more powerful
• Get the information you need to get it done with Firebug.

https://addons.mozilla.org/en-US/firefox/addon/1843

DOM Inspector

Inspects the structure and properties of a window and its contents with a variety of views.

https://addons.mozilla.org/en-US/firefox/addon/6622