1. Tamper data
This is used to view and modify HTTP/HTTPS headers and post parameters.
Using this one can draw out a ruler to get the pixel width and height of any element on a web page.
Colorzilla offers eyedropper, color picker, page zoomer and other tools for
color based development.
Well as the name suggests this add-on reveals the IP of the current page, the ability to copy the Ip onto a clipboard etc
FireFTP is basically a free cross platform FTP client.
this requires firebug and allows web developers and designers to overlay a web composition over top of the developed HTML.
8. Web Developer
This Firefox add-on adds a menu and other tools with various web developer tools.
Firebug has a lot of tools that can be used to do many things