The quality of the translations is not as Google Translate's one, but the fact that everything is done offline in your local computer is a BIG ADVANTAGE. Not only in terms of privacy, but also because webpages translated on this way never crash or have weird behaviors.

The translation bar always present is a bit of overkilling. It's much better to hide it and use the small icon next to the URL.

There are still some things to fine tune and improve, but this is a really promising add-in. I hope it will become standard in Firefox in the future.