Until recently, it was really great, but since the last update, it's become quite annoying. Every time there's a suggestion for correction, two windows pop up simultaneously, which you always have to close with a mouse click, and that's killing my nervs. If you, like me, use a lot of specialized terminology that the language tool doesn't recognize yet, the entire text is constantly plastered with suggestions. It makes working impossible. Additionally, it now only wants to correct into Dutch, even though German and English are selected. This renders the tool completely unusable.

It works well, but you can't exclude the language from fixing.
Also, you can't ignore the same rule in different languages. For example, the rule is that a sentence doesn't start with an uppercase letter. I clicked 'ignore that rule' in English, but in Russian, it still works, and the 'ignore the rule' button doesn't work.

Extension très pratique à la rédaction de formulaire, corrige orthographe et grammaire.

Очень хорошее дополнение для браузера, отлично исправляет ошибки в тексте.

Not very good and over-complicatef

I meant complicated of course but this rubbish app didn't pot th error

If I can't install a different spell-checker on Firefox I'm going to use a different browser, that will be a shame.

Edited to add: Despite the Developer's response my opinion stands that this add-on is useless. Fortunately I have found an alternative that works perfectly.