So here is a list of my favorite Web Design And Development Addons for Firefox, take a look.
Adds a menu and a toolbar with various web developer tools.Download here
Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies...
Draw out a ruler to get the pixel width and height of any elements on a webpage.
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