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.

The fact that I can silence Grammarly while I write and then use it to proofread helps to prevent distractions. I do wish that I could save some exceptions that I have to contend with every time I write. Still, Grammarly is a very useful tool to help sharpen my writing.

I do not have the premium version, yet the free version remains helpful in catching common typos. While I may disagree with some of its suggestions, I find this extension most helpful in ensuring most of my web communication is free of typos and contextual errors. I dislike, however, having to sign-in to the program each web session. What a hassle. Otherwise, a valuable extension.

In short, very good to use. If your grammar and punctuation is a bad as mine then this will be a added bonus to use to help correct spelling mistakes on your application forms and emails and social networking, etc.