Among all the Firefox extensions I regularly use, one of the most useful is ColorZilla. The latest version is ColorZilla 1.0. It really comes in handy while designing websites.
You will need to restart the browser after it has been downloaded. A eyedropper icon will then appear on the left hand corner of your Firefox browser. On clicking the eyedropper icon, your cursor will be visible as a cross. You will then be able to obtain a color reading from any point in your browser by moving the cursor around.
You can also zoom up the page and even measure distance between two points in your browser. Double clicking the eyedropper icon, will load up the built-in palette as a pop-up browser where you can choose from the pre-defined color sets, including web safe colors. There are loads of other stuff you can do with it.