So here is a list of my favorite Web Design And Development Addons for Firefox, take a look.
Web Developer Firefox Add-on
Adds a menu and a toolbar with various web developer tools.Download here
ColorZilla Firefox Add-on
Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies...
Download here
Measure it! Firefox Add-on
Draw out a ruler to get the pixel width and height of any elements on a webpage.
Download here
FireBug Firefox Add-on
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.
Download here
IE View Firefox Add-on
Lets you load pages in IE with a single right-click, or mark certain sites to *always* load in IE. Useful for incompatible pages, or cross-browser testingDownload here
