I found a great free Firefox add-on for taking a screenshot of long scrolling webpages called Page Saver Basic. The add-on places a small camera icon in the upper right hand corner of Firefox. To take a screenshot of the page you’re viewing simply click the icon and it will ask you where to save the image. This beats the hell out of my old method of stitching together a series of print screen captures in Photoshop.
The add-on is also fairly configurable. You can choose between PNG and JPEG file formats. You can also scale the size of the screen capture and select a default filename pattern. Other handy options include a keyboard shortcut, including or excluding the browser’s menus, and capturing the entire page or only the visible portion.