ColorZilla
… is an extension for Mozilla Firefox and the Mozilla Suite.
It assists web developers and graphic designers with color related tasks – both basic and advanced.
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, saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily