How can I take a screenshot of a whole web page?
The GCLI no longer exists. See answer to duplicate question.
On recent versions of Firefox, there is no need for any plugins. This capability is built into the browser.
- In Firefox, use Shift+F2 to open the in-browser CLI. This is a command-line interface at the bottom of the browser window which lets you do all kinds of awesome stuff. (I find the
restartoption particularly useful.) - Type
screenshot --fullpage(this will autocomplete, sosTab--fTab will work; the autocomplete is shown by grey text ahead of the cursor).
That will save the screenshot as a PNG file to your downloads folder. By default, the filename includes the date and time. You can use a different filename if you wish, simply by passing it in on the CLI: screenshot --fullpage my_name.png.
Screengrab does what you want.
Edit: Since screengrab is not maintained anymore, Awesome Screenshot is a good option:
https://addons.mozilla.org/en-US/firefox/addon/awesome-screenshot-capture-/
There is an amazing extension for Firefox called 
pdfit, which allows you to save any webpage you are currectly watching entirely (from top to buttom) in varios formats: pdf, jpg, ...