Thank you for the plugins , some of them same to be really useful to me.
I use the next:
Firebug. 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... Too useful, you can see the source code, change it and see the changes online.
Colorzilla: Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies... I use it to get colors that i like in famous websites.
Web developer: Adds a menu and a toolbar with various web developer tools. I use it to disable css and see if my website is usable.
YSlow: YSlow analyzes web pages and why they're slow based on Yahoo!'s rules for high performance web sites.