It has some interesting ideas for selecting duplicates, like checking all similar bookmarks from a folder, but it quickly becomes messy when dealing with many duplicates across different folders (might work for people with fewer bookmarks, I guess). Unfortunately, this extension has no progress indicator, so there's no way of knowing if [Merge duplicate folders], for example, is really merging folders or just crashing badly, consuming lots of memory and processing power (after several minutes of waiting, I had to restart Firefox and didn't find anything different from before using it...)