http://stackoverflow.com/questions/23742901/take-full-localhost-webpage-screenshot-firefox-windows
I'll answer my own question.
Forget about add-ons. You can do it via the browser's built-in Developer's Toolbar commands.

1. Press Shift+F2 and to bring the toolbar up at the bottom of the screen.
2. Type: screenshot {file name} --fullpage
3. You will be asked for a location to sav your screenshot in .png format (e.g. {file name}.png).

You can also omit the file name if you don't want the image to be saved
and rather send it to the clipboard:
- screenshot --fullpage --clipboard
Type screenshot help to see more interesting options.