Dear arifianace, For the problem, you have two ways to do the screenshot. The first one is right click on an empty part of the page and select "Take Screenshot". Alternatively, use the keyboard shortcut "Ctrl + Shift + S". The second way is customize your Firefox toolbar to add a Screenshot button and below is the link on how to add the button. The screenshot button you can drag next to the menu button but the choice is up to you.

https://support.mozilla.org/en-US/kb/take-screenshots-firefox

Hopefully my solutions can help you solve the problem.

Hi arifianace, if none of the above methods work, please check to make sure Firefox Screenshot isn't disabled:

(1) In a new tab, type or paste about:config in the address bar and press Enter/Return. Click the button accepting the risk.

(2) In the search box in the page, type or paste extensions.screenshots.disabled and pause while the list is filtered

(3) If the preference is bolded and has a value of true, double-click it to restore the default value of false (false means "not disabled")

More info on about:config: Configuration Editor for Firefox.