It is a quite effective and a very helpful tool while writing in two or more languages.

Excellent product. However, not satisfied with the add-on. The inline pop-up closes every time the page is scrolled. My display is 1280x, and I use a mouse, and i will have to scroll to see the language suggestions. It becomes very annoying to reopen often. Need an option to close manually only using the close button, it should not close on page scroll. It also sometimes closes itself, when fetching paraphrasing suggestions, must be a bug.
Update: Posted, and pending.

Looks like very robust alternative to grammarly

très appréciable cette extension, c'est là que l'on s'aperçoit des erreurs que l'on fait en tapant vite.

Un plugin realmente imprescindible. Mejora en mucho los diccionarios integrados en Firefox. Uno de mis TOP 10 plugins.

Очень удобно, когда занят мыслями и не замечаешь, что допустил описку. Особенно, когда пишешь не на своём языке. Разговаривать с чехами по-польски оказалось проще, чем на русском. Дважды мне это очень помогло в жизни.

Indispensable, seas un experto escribiendo o no.

Speaking only to the functioning of this extension I would definitely rate this a 5-star! It is easy to use, easy to ignore, and generally correct.

Eu adorei. Extremamente útil para mim. Converso com várias pessoas e às vezes preciso escrever tudo da melhor maneira possível e a extensão me ajuda bastante. Alguns erros que cometia não cometo mais só por usar ela e saber onde os erros viriam.