Update 2018-07-23
As was just pointed out in the comments, this question was about getting a screenshot from the command line. Sorry, I just read over that. So here is the correct answer:
As of Firefox 57 you can create a screenshot in headless mode like this:
firefox -screenshot https://developer.mozilla.com
Read more in the documentation.
Update 2017-06-15
As of Firefox 55 there is Firefox Screenshots as a more flexible alternative. As of Firefox 57 Screenshots can capture a full page, too.
Original answer
Since Firefox 32 there is also a full page screenshot button in the developer tools (F12). If it is not enabled go to the developer tools settings (gear button) and choose "Take a fullpage screenshot" at the "Available Toolbox Buttons" section.
source: developer.mozilla.org
By default the screenshots are saved in the download directory. This works similar to screenshot --fullpage
in the toolbar.