See this addon, featured by My Digital Life: CheckPlaces.
sometimes it is quite messy especially you have a lot of bookmarks with some of them duplicated, broken links or links no longer valid. Now, there is a Firefox extension, named as CheckPlaces that can help you manage and clean up your bookmark in efficient way.
Notice at the top, under
Checked, it has some statistics regarding your bookmark collection:
According to the CheckPlaces addon page at Mozilla Addons, it is compatible with Firefox Beta 4.