Excellent! Rated 4 out of 5 stars
It works a lot better than the default Firefox spellcheck, but there are some concerns.
Firstly, you have to have an account. You can get one for free, although the will still have access to your personal information. There is no reason for a company to require you to have an account to use a free product, other than tracking.
Also, they collect your data and sell it to advertisers. This includes everything you type. I have yet to find out whether or not that includes passwords, but I would assume it does.
Finally, it's a bit slower than other programs.
Overall, I think it's a really good add-on. Impossible to find a better alternative.