ScreenGrab Firefox Extension
A great Firefox extension for taking screenshots is called ScreenGrab. ScreenGrab will take screenshots of entire web pages — even the parts that run off the screen.
To take a screenshot of a web page in Firefox with ScreenGrab, just right click on a web page and choose: ScreenGrab! —> Save document as image. Give the extension a few seconds to startup and take the screenshot. When ScreenGrab is done it will open a save dialog box where you can choose a filename for your screenshot.