Firefox: taking fullscreen screenshot

Steps:
1. press F12 to bring up Web Developer Tools
2. press F1 to open up Settings
3. scroll down to Available Toolbox Buttons and enable Take a screenshot of the entire page
4. a camera icon would appear at the top right of the Web Developer Tools panel. By default, it saves the screenshot in your Downloads directory.

Ref:
Firefox 32 for developers - Mozilla | MDN

For Chrome, see here