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LanguageTool is a free grammar checker and paraphraser for English, Spanish, and 20 other languages. Instantly check your text for grammar and style mistakes.
I like it very much and thanks to the various extensions it’s also trivial to use in Firefox and Libreoffice.
It works on Firefox/Fennec/other forks on Android.
At last the browser extension is closed source. I don’t know the others
https://beehaw.org/comment/893089