ColorZilla is a free Mozilla Firefox and the Mozilla Suite add-on. It helps web developers and graphic designers with color related tasks such as get a color reading from any point in your browser, quickly adjust this color and paste it into another program,Zoom the page you are viewing and measure distances between any two points on the page,choosing colors from pre-defined color sets and saving the most used colors in custom palettes. DOM spying features allow getting various information about DOM elements quickly and easily.
Download ColorZilla from above link and restart your firefox.
To open ColorZilla options,
Go to Tools menu in firefox and click ColorZilla.