You can use the Developer Tools to take a screenshot of the entire page, or of a single element in the page.
Taking a screenshot of the page
Use the screenshot icon: 
to take a full-page screenshot of the current page.
By default, the screenshot icon is not enabled. To enable it:
- visit the Settings page
- find the section labeled "Available Toolbox Buttons"
- check the box labeled "Take a screenshot of the entire page".
You'll now see the icon in the toolbar:
Click the icon to take a screenshot of the current page. The screenshot is saved to your browser's "Downloads" directory:
Taking a screenshot of an element
To take a screenshot of a single element in the page, activate the context menu on that element in the Inspector's HTML pane, and select "Screenshot Node". The screenshot is saved to the browser's "Downloads" directory:
Copying screenshots to the clipboard
From Firefox 53, you can also copy the screenshot to the clipboard. Just check the box in Settings labeled "Screenshot to clipboard":
Now, whenever you take a screenshot, the screenshot is also copied to the clipboard.