I am not native English speaker so my English isn’t perfect.
I found this extension called Grammarly which supports Google Chrome & Firefox.
It works in any textbox on the sites and you can disable it for specific sites. The correction is quite good.
If you want you can grab Grammarly Premium for FREE for a limited time via https://gram.ly/FNjQ
How well Grammarly works you can see in this video preview.