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.
Fortunately, browsers can help us out a bit here.
As of Chrome 59, it's built into DevTools. Here's a video. You use "Responsive Design Mode", then the menu option to get the full page screenshot is in the menu in the upper right.
I find it easy to use the command prompt to do it as well:
If you need a "mobile" full-length screenshot, just adjust the responsive view to the size you want and save again. Handy!
I've also had good luck with the Nimbus extension in Chrome.
There is a setting in the Firefox DevTools that you need to turn on called Take a screenshot of the entire page under Available Toolbox Buttons. Flip that on, and you get a button.
In Safari DevTools, select a node, right-click, and you'll see Capture Screenshot in the context menu.
Safari has File > Export as PDF, but it's pretty awkward. I have no idea how it decides what to export and what not to, the layout is weird, and it's broken into multiple pages for some reason.
The Awesome Screenshot extension seems to do the trick.