I’ve use many firefox plugins and the followings are my best favorite plugins for web developer.
Web Developer Extension (http://chrispederick.com/work/firefox/webdeveloper/)
The Web Developer extension for Mozilla Firefox and Mozilla adds a menu and a toolbar to the browser with various web developer tools.
LiveHTTPHeaders (http://livehttpheaders.mozdev.org/)
Help debugging web application.See which kind of web server the remote site is using. See the cookies sent by remote site.
MeasureIt (https://addons.mozilla.org/extensions/moreinfo.php?id=539)
Draw out a ruler to get the pixel width and height of any elements on a webpage.
ColorZilla (https://addons.mozilla.org/extensions/moreinfo.php?id=271)
Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies.