Snap happy
Firefox has a buried feature in its Developer Tools, which is potentially useful even if you're not a developer. If you've never used the Developer Tools, just press F12 and you'll see the tools.
You should see a cog icon towards the upper right of the pane that's just displayed. The cog is on the same row as the Inspector, Console etc. buttons.
Clicking this displays a page of options that you can use to customise the Developer Tools. By ticking "Take a fullpage screenshot" under "Available Toolbox Buttons", you should see a camera icon appear near the cog icon you previously clicked. Selecting this gives you a screenshot of the page where you opened the Developer Tools. The resulting PNG file can be found in your Firefox Downloads.
This is a really handy feature. It means that you can take an image of a scrollable page that's larger than your current window. No need for screenshotting parts of the page then joining them together by hand.
Solving bigger problems
But what if you want to enlarge the visible area beyond the size of your current window, and then take a screenshot?
Why would this be even useful?
Let's say that you want a screenshot of a web page displaying a map, where the map expands to fill your browser window. And let's say that you want the screenshot to cover a larger area than fits into the largest browser window you can have on your desktop. Just expanding the window isn't going to give you the size that you need.
How can you do this? If you hover over the toolbox icons in the Developer Tools panel, near to the cog, you should be able to find the Responsive Design mode. (It's a very small, dotted rectangle inside a larger, thicker rectangle.) Click this. Your display will change to have a border and extra options appear. The selected resolution is shown above the display. Clicking on the resolution dropdown menu, you can directly type the resolution you want this preview display to have and press Enter to have it take effect.
All that's left is to take a screenshot again using the "Take a fullpage screenshot" option in the Developer Tools toolbox. (Note that Responsive Design mode has its own screenshot tool, near to the resolution menu, but this only takes an image of the page visible within the display window you've just set, i.e. it's not necessarily the full page if there would still be scroll bars at that actual display resolution you've selected.)