You are watching live animals full screen on YouTube and you want to capture a screenshot? The Windows PrintScreen (or PrtScn) key will capture the entire monitor display to the clipboard for easy pasting into an image editor or document. However, that doesn't help if you want to stay within the video viewer because Windows 7 doesn't auto-save to disk. You might look for a Windows program (outside of Firefox) for this because Firefox add-ons also aren't allowed to save directly to disk without your approval, and that would interrupt your viewing experience. This one sounds interesting, you can assign a key that will capture and auto-save the screenshot. I haven't tried it myself: https://superuser.com/questions/412293/how-to-take-a-screenshot-on-windows-7-and-auto-create-screenshot-file-on-desktop/412302#412302 (Screenpresso)