2) Web Developer Toolbar : Adds a menu and a toolbar with various web developer tools. It has various options which the developers will love to use.
3) Colorzilla : With ColorZilla you can get a color reading from any point in your browser, quickly adjust this color and paste it into another program. You can Zoom the page you are viewing and measure distances between any two points on the page. The built-in palette browser allows choosing colors from pre-defined color sets and saving the most used colors in custom palettes.
4) IE Tab : This is a great tool for web developers, since you can easily see how your web page displayed in IE with just one click and then switch back to Firefox.