It is a useful tool, but not the best of its kind. Using it for years, but random bugs and issues are killing me. It is not about hoping to fix; those are part of Grammarly.
Also, it seriously struggles with Gmail, and god forbid to use it in Outlook. It does not show or underline irrelevant things that you must click somewhere else to open them.

The addon is good, but it causes UI freezing, especially on MacOS, and to fix that I need to switch to other application and then back to FF.

Turns off by itself many times.

I like it but it gets in my face too much. Most of the time, I don't care about my grammar because it's just some informal thing. I do use it when I want to be taken seriously, like when I complain to my congressperson about some bone-headed thing they did or didn't do.

Is not working fine in Firefox. Always appear as logout... I´ve logged in many times and always stay logout...

This app is excellent in grammar checking but would be a higher rating if the yearly cost would go down for a subscription.

Not this runs on many but not all combinations of browser and websites so I also use Language Tool. Some of the features get in the way but these tools are both very good.

It really needs an option to be turned off by default and only turn on on the sites I need it.

Works well enough. I uninstalled it because I found its built-in ads for Grammarly premium to be annoying and mean-spirited. It pops up in emails saying something like, "Are you SURE you want to send this email with its ADVANCED ISSUES, like poor word choice and weak phrasing? Click here to try Grammarly Premium!" Like, go climb a tree, Grammarly. You're here to correct typos, not neg me into shelling out cash over non-existent "advanced issues" in perfectly good emails. Obnoxious app.

It suggests improvements on Google Docs documents, but not able to make changes. Seems a bug.