I'm leaving three stars at least as this is a great extension, and I've found it to be reliably accurate as far as free online translators go. Not as good as accurately learning the language yourself, but much more accessible.

That said, as of Firefox 49, this extension no longer works. I wouldn't know why, it's there in the plugin bar, you can put text in it and change languages, but it never translates. I've also just confirmed this is still the case with the recent update FF50.

As I've said, great extension if you need quick translations online, but for now you're going to have to downgrade to Firefox 48 and stick with it (and deal with Firefox chewing you out several times a day for not updating).