I thought that this was a great grammar tool, but when I Started watching a video, The video would just not appear. And when I removed Grammarly, The video worked absolutely fine. I was not signed in, so My guess is that If you don't sign up/in, they will break the website you are on.

When it works it works very well. About one half the time it signs it's self out. Doesn't pick up mistakes on Facebook. Adds commas where they don't belong.

Very inaccurate, but one of the better ones though which doesnt say much. they all are sheet.

This add-on insists on creating and storing nasty cookies. It interferes with adblockers too. It provides slightly better spell check results than FF which is pathetically useless. How could FF have such a poor native spell check?

Since there's no way for this add-on can't run without you first connecting and accepting cookies from your gmail account, it's uninstalled.

Grammarly provides an excellent help in composing the written document to ones satisfaction.