Eric Meyer points out a great little utility for taking screengrabs, screen-sized or full-page, using one of the two consoles built into FireFox’s included Developer Tools.
Essential Tool: Firefox’s screenshot Command
Update: With FireFox 67 (and likely a few previous versions), a screenshot tool is available off the drop-down menu that appears when you click the ellipsis (…) at the left end of the address textbox. Options include a full-page save, the visible window save, or a region of the screen.