If you are on Firefox/Waterfox, use Screengrab (free). It offers a choice of screen shoting the complete page, a selection, or just what's visible in the window at the time. Also, it doesn't take a shot of the browser tabs if you don't want it to. If you're on Chrome, use Screencapture, which not only takes screenshots, but also allows you to "edit' a live page before taking the shot.