If you use Firefox and you are a designer, you should be using ColorZilla, which is an add-on for Firefox. ColorZilla adds an eyedropper icon into the status bar at the bottom of your browser. When you click on it, the arrow turns into a crosshairs while you move it around on the screen. The cool part is that it displays the hexadecimal values in the status bar for whatever color the item is you are hovering on. In addition, it displays the RGB values, plus the CSS selector for the item.
It’s very useful because it will tell you color values right from from your browser, and it even works on images and backgrounds. There are many times when I want to color match something, or I am too lazy to fire up Photoshop and use the eyedropper tool. It really is a huge time saver. So, download ColorZilla today and get using it, if you aren’t already.