Kelsey Ruger has summed up some pretty neat firefox extensions for web development on his blog. If you use Firefox, go get some of them so that your Firefox actually can become useful.

Colorzilla - An eyedropper tool for color selection and testing. Hovering over an element shows color info, DOM path info, and the X, Y offset from the previous click.

Fang - This extension creates a textual representation of a web page similar to how the page would be read by a modern screen reader.

http://www.themoleskin.com/archives/10-firefox-extensions-for-web-development/