Idea is great, execution is terrible. When attempting to sync across FF and Chrome (the ONLY reason I installed this), it locks up both in Firefox and Chrome. It also screws up the bookmarks when uploading/downloading and puts them into the different browsers in different locations/orders, it gets confused by bookmarks in folders, and to sync more than a thousand bookmarks, they want $30! Dishonest and terrible, I could not get them to give me a refund when it did not work.
Additional: "back up your favorites" ummm Firefox DOES THIS. By itself. Without any extensions. You just have to log into a FF account.