I had a similar experience as Loquacious1 and Anonymous user 436dc3. I installed Bookmarks Organizer and the first time I ran it, the three balls kept cycling. The second time I tried, it ran to completion. However, the check bookmarks remained greyed out and it was unresponsive.
I uninstalled the Add-on. I downloaded the Add-on again. I reinstalled. It runs fine.
Can't say that this was the first time that a reinstall cleared up the problem. Been around since the NCR 300 series...