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.mozilla.org/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/

Garbage, it says it screenshots the whole website but doesn't do that. Pages with a scroll bar still only get partially screenshotted.

Seems that unless you scroll down to the bottom of the page, it will not capture accurately.
The feeback button does not work. URL never resolves.
Captures with inconsistent results.

Mozilla should be ashamed that a subsidiary trading on the good name of Mozilla is peddling this crap, shame on you.

Worthless.

Can't turn off sound when save the screenshot. It's disturbed, please fix.

Useless. It doesnt give the option of where to save and does not put it in 'Desktop' as claimed.

Why can't add it to my Firefox?

Não consegui tirar uma captura de tela de página inteira, que possui o seu principal conteúdo em uma longa "sobreposição" (overlay). Infelizmente, a sobreposição é cortada após a primeira página (ou após a primeira parte visível na tela), e fica incompleta. A ferramenta nativa "Capturar tela" apresenta o mesmo resultado. Site: http://www.arquivoestado.sp.gov.br/site/acervo/memoria_do_imigrante/pesquisa_livros_hospedaria ; Arquivo da captura nativa do Firefox: https://postimg.cc/xk8ypWdv ; Arquivo da captura da extensão Awesome Screenshot Plus: https://postimg.cc/bDQGmT10 ; Esta captura da extensão concorrente também não está elogiável, porque não está perfeitamente contínua, porém, prova que a captura da Mozilla ignorou uma parte considerável da sobreposição, que neste caso possui a informação mais importante. Espero que leiam a minha mensagem e aprimorem o recurso.

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

Kiváló! Teszi a feladatát. Gyors, egyszerű.