This add-on is terrible. For four years I have been struggling with cleaning up my bookmarks and it was making my my live real hell. Imagine that you have 500 bookmarks. After installing this one, out of the blue I had 1800 bookmarks. Looks like it is doing one way synchronizationfrom Safari to FF, not synchronizing back bookmarks from FF to Safari. Each time I cleaned up my bookmarks on FF this add-on was putting old, deleted and those moved someware else bookmarks back in old places. Apple is making so low quality software these days...