This used to be a good add-on, but last year they redesigned it to be basically non-functional unless you register for their services.
It also vastly slows down browsing; for me it was adding 10-15 seconds for EVERY page load, and significantly slowing down browsing within pages. This was going on for months, which I attributed to browser updates, but after removing this specific extension the problem was immediately resolved.

You can see in older reviews that they keep claiming this was a bug that they've fixed, which is clearly not true.
Do not install this. If you still have it installed from back when it was worth using, the sooner you get rid of it the better.