Try going to the Firefox configuration screen by entering about:config in the Firefox address bar. Then search for extensions.screenshots.disabled and make sure it's set to false. Also, do the same for extensions.screenshots.system-disabled, although that preference may not be appear on every Firefox browser.

If you have to change the setting, restart Firefox for the changes to take effect.

Hope this helps.

You can find items like "Take a Screenshot" and the bookmarks star in the drop-down list that opens if you click the "Page actions" button (3-dot icon at the right end of the location/address bar). You can right-click an item in the "Page actions" drop-down list to add its button to the location/address bar for easy access.