The last update for Smart Bookmarks Bar was in February 17, 2009!
There is an add-on, offering identical feature, called "Roomy Bookmarks Toolbar". People here might call it a rip-off, but it works with FF4.
Don't get me wrong, I love Smart Bookmarks Bar (Been using it for years), but move on people.
Ps. I will be back when Smart Bookmarks Bar get an update!