Web developer or designers don’t only need to check their creations in Firefox but they mostly also love the browser for the simple fact that there are a ton of handy plugins available for that particular browser that makes their effort easier. The last half year I also discovered quite some interesting plugins so I thought it would be nice to share those.
Web developer toolbar. Adds a menu and a toolbar with various web developer tools.
YSlow. Integrates into Firebug. YSlow analyzes web pages and tells you why they're slow based on Yahoo's rules for high performance web sites.
Firescope. FireScope is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS. This is a nice tool that can help you out to search the html and css archive from the known site sitepoint.com.
Colorzilla. Advanced Eyedropper, ColorPicker, Page Zoomer and other colorful goodies...
Measureit. Draw out a ruler to get the pixel width and height of any elements on a webpage.
IE Tab. Switch to IE from within Firefox. Very handy to see how your page will be displayed in IE.
Cacheviewer. This extenion is GUI Front-end of "about:cache". Allows searching and sorting memory and disk cache files.