It offers very good functionality, however almost no configuration options. The plugin runs by default on all pages resulting in huge performance impact, severely increased memory use for Firefox and indeed privacy concerns.
Have you just started to type something sensitive on a page without remembering to disable Grammarly first? Bad luck, your data is already on their servers.
Setting the plugin to be disabled by default and only enabled on demand would be absolutely necessary. The way it is now, is shady and too pushy.
I can see how they want it to run everywhere, but still, this is a big no-no, to by default automatically send all my typing to a remote server.
Also the resource usage is really bad. If you use few tabs and restart browser every offten, probably it's fine, but I usually have several tens of tabs in variety of windows and Firefox running in one go for a week. Now, if you are such a user, disable this, and you'll be amazed how much better the overall responsiveness of Firefox will become.
Long story short: it offers nice feature, but the cost cannot be justified.
I'd just go to https://app.grammarly.com/ if I need writing assistance, and keep away from this plugin for good.

Two years ago, my review was not very positive. Since then, however, this addon has improved a lot. Now, it can open a full window for the contents of each text box. It provides a comfortable environment for writing large chunks of text and provides tips on spelling, grammar, and pragmatics. A correction is always recommended.

One of my objections was that Grammarly did not work in Wikipedia, OneDrive, Word Online or Google Docs. That's where I needed it the most. It appears the Grammarly developer team has taken steps to resolve this problem too. Integrating Grammarly with said web apps is not possible. But now, it is possible to import your documents into Grammarly's web app and work on them there.

Wikipedia is still out of this addon's reach, but quite frankly, screw Wikipedia. It is a horrible place where editors would kill each other if only they could!

At 76 my spelling an punctuation is suffering from keyboard stress-- Grammarly helps a lot :)

tremenda herramienta para evitar cometer errores al escribir y llevar el mensaje correctamente.Gracias.

Garbag dosnt work with firefox dosent highlight spelling errors only the built in firefox one works but is useless unless you have a slight type-o

Always uses it, find little misspelled words sometimes.