Solved! - Type about:config in the address bar and press Enter. - A warning page may appear. Click Accept the Risk and Continue to go to the about:config page. - Type extensions.screenshots.disabled in the Search field. - The extensions.screenshots.disabled preference displays. - Click the Toggle Fx71aboutconfig-ToggleButton button next to this preference to set it to false.

However, I also have no idea why the function was suddenly set to "true" by default. This should be changed!

That was quick work.

The default hasn't changed, but this glitch seems to happen to some users on every update. I don't know what causes it, but perhaps there is a timing issue when Firefox is checking whether Firefox Screenshot -- which is packaged as an add-on and placed in a subfolder -- is present. ??