As a grammar checker it's really good. My biggest problem is how it collides with other UI aspects of the interface.

For text boxes that are expandable you grab the bottom right of the box and drag it to a bigger size.

Grammarly puts its interface in the same area but further in the text box. 80% of the time I can expand the text box fine, but there are times when Grammarly just grabs focus and there is no way for me to expand the text box.

I have sent this to them as a bug report months ago but heard nothing back. I have had this issue on both Windows and Linux installs of Firefox.