Used this religiously back in the old days, and back with Waterfox. I've come back to Firefox Quantum 65, and while the addon works in 'guest mode', it does not register logging in to the Grammarly website.
The support staff has completely bent over backwards to try and help (thanks Irene and Tiffany!), but to no avail. I hope people have better luck with this than I have.

perfect, fast, free

best thing that happened in my life

xirotera

Grammarly is active, but
key features are missing

Great tool when you are not a native English speaker.

it comes in handy