As you may already know, starting with Firefox 54 the browser comes with a new Screenshots feature which allows the user to capture an opened web page and save it to a file or upload for sharing. In this article, we will see how to enable the Screenshot toolbar button in Firefox for faster access to this feature.
Firefox Screenshots is a new system add-on. It allows you to capture a screenshot of the opened web page and quickly share it with your friends. The screenshot can be uploaded online and shared over Facebook, Twitter and Pinterest. We have reviewed this feature in detail here: Firefox Is Getting The Screenshots Feature.
The server-side software of Firefox Screenshots is open source. Interested users can install it on their own hardware.
To enable the screenshot button in Firefox 55, do the following.
- Open a new tab and enter the following text in the address bar:
about:config
Confirm that you will be careful if a warning message appears for you.
- Enter the following text in the search box:
extensions.screenshots.system-disabled
- Set the extensions.screenshots.system-disabled value to false. In my case, its default value was "true".
This will enable the screenshot button instantly. It will appear on the toolbar.
You can move it to the browser's menu (right-click the button to move it) or re-arrange buttons in the "customize" mode. 
Click on the menu button and click the "Customize" item at the bottom of the menu pane. 
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No scroll web page function?
I have not tried yet. I will try when return to Windows.
Firefox Screenshots, formerly known as a Test Pilot Experiment PageShot, is now available in Firefox 55 Nightly as a feature. The screenshots feature, lets you capture web pages and save to your computer or share on social platforms, has been added as a system add-on to Nightly version, but disabled by default.
Firefox 55 now is in the stable branch.
Just tried and I think it’s still a useless feature.
You can’t:
-take full page (scroll) screenshots
-save screenshots in another format different from .png
-choose folder to which save screenshots.
I haven’t tried yet but for me there was an option to take a screenshot of whol3 page (with thebparts that aren’t currently shown in screen included). Yet, I agree with the folder choosing part.