I thought Grammarly would make me lazy. On the contrary. After about one month of use I found that I no longer make mistakes worthy of Grammarly corrections! I do appreciate the automatic typo correction that is not your normal auto-correct.
I was great before and now I am an expert.
Worth EVERY penny.

Latest reiteration continuously adds it's cookie onto your PC even when not in use, so it's tracking every move of yours. This is a huge privacy issue. Uninstalled.

Terrible performance hog - it slows Firefox to crawl, makes it practically unusable for browsing the web.

It was quite a good plugin until they start to label people based on their nationality, race, and color.

Granted this add-on has bugs, it's still extremely useful for me since I have a nasty habit of pressing send before realizing I mistyped one word in the whole paragraph.

The added benefit of being able to go to the website via the extension is a nice short cut. The website has a few more features that comes in handy, but the add on alone has:

spell check/correct
mood/tone
grammar check
btw, I'm using the free version.

As for the bugs. Sometimes the words don't auto correct, but add the correct writing at the end of the misspelled word. The add-on doesn't work on all websites or documents. The logo for the add-on appears in the bottom right of all applicable word boxes, which can also be where an important button may be (like the send or emoji button.)