It can be quite useful to get a "full page" screenshot in a browser. That is, not just the visible area. The visible area is pretty easy to get just by screenshotting the screen. A full page screenshot captures the entire website even if it needs to be scrolled around to see all of it. You could take individual screenshots of the visible area and use a photo editing program to stitch them together, but that's a pain in the but. Nevermind the fact that it's extra tricky with things like fixed position elements.
There is a setting in the Firefox DevTools that you need to turn on called Take a screenshot of the entire page under Dallas Stars Dallas Stars Jersey Available Toolbox Buttons. Flip that on, and you get a button.