If you use the program browsing Firefox and updated it to 55.0 version, you may have noticed the new "Shot Page" icon in the browser toolbar.
This icon authorizes Firefox to copy one screenshot that interests you. If you don't see it, it's because it's gradually rolling out to users of the new Firefox.
This does not mean, however, that the screenshots tool is not already available in your browser. There is but you need to enable it before you can use it. Those of you already see it, better read something else.
Note: Make sure you are using the Firefox 55 update. You can check the version of the program from:
about: support
Let's see how activation can take place:
Open the following address in your browser and accept that you do not care if you lose the warranty: 
about: config? filter =extensions.screenshot
You will see two different parameters: 
Extensions.screenshots.disabled – This is the main parameter. The value false means that the function screen is enabled, while true is disabled.
Expansions.screenshots.system disabled - The Mozilla Foundation uses this parameter to control how screenshots work. If set to true, the display function is disabled regardless of what is set in Extensions.screenshots.disabled. If set to false, the screenshot feature is enabled if Extensions.screenshots.disabled is set to false.
To disable the screenshot functionality in Firefox, set the extension.screenshots.disabled to true.
To enable the screenshot functionality in Firefox, set Expansions.screenshots.system disabled to false and extension.screenshots.disabled to false as well.
You can change to true and false by double-clicking on the parameter of your choice.
To remove the new Firefox tool from your browser's address bar, right-click on the icon and choose to remove it from the toolbar.
To get a screenshot with your browser, click the icon to activate the feature. The display tool supports downloading an area, the visible part of a web page or the entire page.
You can save screen shots on your system or upload them to screenshots.firefox.com to share them online. The Mozilla Foundation keeps screenshots online for two weeks by default, but you can delete them at any time, or even expand the storage date.
