Straight up DOES NOTHING on Firefox Nightly 60.0a1 (2018-02-24) (Hasn't worked before this version either)
When I click the icon it always says "Grammarly is active, but
key features are missing" and prompts to sign in. When I click on sign in it just opens the Grammarly page showing that I AM signed in.
That is the only place where it works.

Update:
Still the case on Firefox Nightly 68.0a1 (2019-04-12) (32-bit).
The grammarly icon on text fields is there, but it never seems to figure anything out. Trying to sign in via that icon doesn't work either. After putting in my login details and pressing "login" the button gets a spinning loading icon that spins for a while then stops. Nothing happens.

I'll be using "LanguageTool" instead. At least that works.