Take a full page screenshot with Firefox on the command-line

The Developer Toolbar GCLI and Shift+F2 shortcut were removed in Firefox version 60. To take a screenshot in 60 or newer:

• press Ctrl+Shift+K to open the developer console (⌥ Option+⌘ Command+K on macOS)
• type :screenshot or :screenshot --fullpage

Find out more regarding screenshots and other features

For Firefox versions < 60:

Press Shift+F2 or go to Tools > Web Developer > Developer Toolbar to open a command line. Write:

screenshot

and press Enter in order to take a screenshot.

To fully answer the question, you can even save the whole page, not only the visible part of it:

screenshot --fullpage

And to copy the screenshot to clipboard, use --clipboard option:

screenshot --clipboard --fullpage

Firefox 18 changes the way arguments are passed to commands, you have to add "--" before them.

Firefox 88.0 has a new method for taking screenshots. If extensions.screenshots.disabled is set to false in about:config, you can right-click the screen and select Take Screenshot. There's also a screenshot menu button you can add to your menu via customization.

You can find some documentation and the full list of commands here.

_{PS. The screenshots are saved into the downloads directory by default.}

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.

I ended up coding a custom solution (Firefox extension) that does this. I think by the time I developed it, the commandline mentioned in enreas wasn't there.

The Firefox extension is CmdShots. It's a good option if you need finer degree of control over the process of taking the screenshot (or you want to do some HTML/JS modifications and image processing).

You can use it and abuse it. I decided to keep it unlicensed, so you are free to play with it as you want.