Used to be good, but now is constantly advertising "premium" features which were standard. Using LanguageTool now
https://addons.mozilla.org/en-US/firefox/addon/languagetool/

This always corrects My spelling and it is helping me understand how to write more. The only thing i don't like is when rarely I write in another language it wants to correct it. Anyways 10/10 would recomend people who struggle with writing or are learning

I always use this when I write.

Used to like it, but with "premium suggestions" that can't be disabled it turned from helpful to insufferably annoying. Switching to LanguageTool, hope it'll be better. If not - I'll be better off without a grammar checker than with something that just doesn't like my writing style and won't shut up.

Only a 1 star for now because if your using dark mode, the red line for misspelled words doesn't show up anymore. :(

To note I'm using the extension Dark Reader and FFs own Dark Mode.

works, but much worse privacy than the mozilla's recommended languagetool addon (https://addons.mozilla.org/en-US/firefox/addon/languagetool/)

Pretty much all spellchecker extensions now suffer from this problem, but about half of the suggestions I get nowadays are not helpful and about one-quarter are simply wrong.

Paid extension after a while.