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CloseReview for Grammar and Spell Checker — LanguageTool by robsku
Rated 4 out of 5 stars
I originally tried Grammarly, and while it did work just fine, this doesn't nag me for money to get the full features and quite honestly just looks and feels more pleasing to use.
There may be some things Grammarly does better, like how it can try to determine the tone of your writing might feel to a reader, but honestly I'm not missing anything. Btw, I don't know if it's even available for Thunderbird, I tried both of these first in FireFox and would recommend this.
The only thing I would personally need for giving 5 stars would be Finnish language support — and a quick way to switch between the two languages I use. But I write maybe like 95-99% text on web, and even offline in English, but Finnish is still my native language, so... ;)
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