I've tested the add-on under Manjaro Linux, and it works for me.

But why would I need it, when Firefox already has a screenshots feature?
https://screenshots.firefox.com/

The "Feedback" link takes me to a Chinese website.
In hindsight, this makes sense, because "Mozilla Online is an organization that operates in China and is a wholly owned subsidiary of the Mozilla Corporation."
However, I was expecting an English website, because the AMO page itself is in English, at least when I look at it:
https://addons.vaultpit.io/de/firefox/addon/easyscreenshot/

It's also strange that the AMO page doesn't have a link to the add-on's Github page:
https://github.com/MozillaOnline/easyscreenshot

Looks like someone didn't read Mozilla's "best practices" guide:
https://extensionworkshop.com/documentation/develop/onboard-upboard-offboard-users/

Why can't add it to my Firefox?

Вроде бы неплохо,удобно и быстро) Пользуюсь нечасто,иногда делаю скриншоты кадров из аниме :D

Kiváló! Teszi a feladatát. Gyors, egyszerű.

t

Its Awesome

Simple and powerful, easy to use. Some extensions may be even copied this one.

valiosísima muy útil, indispensable.