FireShot is normally a 5 star extension. But why has it become incompatible with earlier version of Firefox and clones (Waterfox, Palemoon, Basilik...) that are still used as Firefox Quantum does not support many essential XUL extensions.

I update my post as I eventually found a link to retrieve previous versions of FireShot, the latest compatible with non-Quantum Firefox:
https://addons.mozilla.org/fr/firefox/addon/fireshot/versions/?page=1#version-0.98.93
(Disable auto-update in the Options)