I have the same impression as Larry...
Grammarly is a Gremlin Rated 1 out of 5 stars
by Larry on January 25, 2017.
"How do I get rid of this annoying add-on? ...Next uninstall will be Mozilla".

Well, I won't throw the baby out with the bathwater. I intend to keep using Firefox; however, this Add-on has a nag popup that opens another browser tab directly to its home page each time I open Firefox to go to the web- perfect right? Just what anyone would want and expect. This is what I see attached to the end of Grammarly's home address in the rogue tab's URL address bar: "...sign up?page=free extension _install=true breadcrumbs=true"

This just won't go away. Can anyone help me with this annoyance? I like the Add-on. It is a very good tool but I am about ready to dump it.