Firefox can be one of your most powerful web developing tools. Plugins for FireFox are easy to use and can cut down time coding by hours, check out some of my favorites.
Edit CSS on a page instantly, validate HTML/CSS/XML, clear cache instantly, view stylsheet information and much much more.
Measure any area on a page in px with a click and drag of your mouse, comes very much in handy.
Add an FTP client straight to your FireFox browser.
Use tamperdata to view and modify HTTP/HTTPS headers and post parameters. Trace and time http response/requests. Security test web applications by modifying POST parameters
Ever wanted to test a page you built for the iPhone? The User Agent Switcher extension adds a menu and a toolbar button to switch the user agent of the browser.
Other cool plugins
FoxTab is a 3D tabbed browsing extension. With 5 attractive layouts to choose from, fliiping between opened tabs becomes easy and enjoyable than ever. Use the grouping and filtering features, and start experiencing tabbed-browsing in a whole new way.
Stay tuned for more plugins as I find them