Downloading the add-on only takes a while. You need to restart your firefox window after downloading. After restarting firefox, you'll see a small icon appear at the right bottom corner of your window. That's where you click to save or copy the screen. You can choose to save the whole view of the screen, window, or just your desired selection area. When you're finished with the selection, a save dialog box will appear. Type in the name and click Save. Click here to download.N.B. You may choose PNG or JPG as the image format.