You can also press : or Shift+F2 to open the Graphic Command Line then type in
screenshot filename to take a screenshot. To take a whole page, use
:screenshot filename -fullpage.
You can also visit the help page via
help screenshot. Below is what it shows:
Synopsis: screenshot [filename] [--clipboard] [--imgur] [--delay ...] [--dpr ...] [--fullpage] [--selector ...] [--chrome]
Save a PNG image of the entire visible window (optionally after a delay)
[filename] (string, helpManOptional)
The name of the file (should have a '.png' extension) to which we write the screenshot.
[--clipboard] (boolean, required)
True if you want to copy the screenshot instead of saving it to a file.
[--imgur] (boolean, required)
Use if you want to upload to imgur.com instead of saving to disk
[--delay ...] (number, helpManOptional)
The time to wait (in seconds) before the screenshot is taken
[--dpr ...] (number, helpManOptional)
The device pixel ratio to use when taking the screenshot
[--fullpage] (boolean, required)
True if the screenshot should also include parts of the webpage which are outside the current scrolled bounds.
[--selector ...] (remote, helpManOptional)
A CSS selector for use with document.querySelector which identifies a single element
[--chrome] (boolean, required)
True if you want to take the screenshot of the Firefox window rather than the web page's content window.
Note: this only works for Firefox 61-