LanguageTool works well for me - it is less obtrusive than tools I've used in the past yet enables me to ensure my posts are better than they would be otherwise. Features such as the ability to add unusual words, acronyms, or names to my local dictionary or ignore them, and the ability to navigate to the context of a typo, and also, to ignore suggestions easily if I want to. I have not yet tried a language other than English. I have been especially grateful for LT when I have been unaware a key or two on my keyboard has stopped working. How terrific to see the LT prompt when about to post a quick note in which all letter 'u's would have been omitted. Many thanks to LTers for providing stronger support for privacy and developing a convenient and effective tool!

Excelente complemento, aplica en casí en todas las páginas

Simplesmente incrível.

Brings back the spell check support for multiple languages along with additional features that are of help. Great. (I used a builtin mechanism before but it doesn't work anymore since Ff80 with for instance MS web apps)

Corretor que realmente corrige e funciona com outras línguas também, meu inglês capenga agradece.

Only grammar checker that works with the latest Firefox versions.

Does the job. Gives no trouble. 👍

Спасибо, очень хороший корректор грамматики! Пользуюсь им и очень доволен. Рекомендую всем, кто его ещё не попробовал. Удобно пользоваться и облегчает написание правильных текстов.

Works better than any other grammar-fixer I've tried, helps a lot with typos and mistakes humans tend to overlook

a great add-on. I subscribe in order to get the grammar checking. I'm a native english speaker, I still find it very handy.