The Grammarly addon is broken for WordPress.com online editor. I have a really bad experience on Grammarly ever since I shifted to Quantum from Chrome. On Chrome it worked just fine, but here it needs more work. Hope the developers patch this.

You need to make an account to use it, and the extension also tracks you, placing ads on web pages that it never tells you about. Still, the actual spell checker is good.

When you select English UK. American words like Color and favor and organize. are not detected as error.

Not that great. I was writing a paper for a theatre class, and it told me that I needed to change all instances of "theatre" for "theater." It doesn't really help any more than any built in grammar device anyways, at least the free version doesn't.

Always need to login after every time opening of Firefox. Sometime shows error free, sometimes shows error for some same sentence. After selecting their suggested correction, it pastes the whole sentences again. So disturbing. I can't found any problem in the Chrome edition though.