This addon really needs to be removed from the directory. It hasn’t been updated since 2007 and is not compatible with any “modern” version of Firefox.
It’s the first result I see in a Google search for a bookmark link checker, which is a problem.
Old add-ons will be delisted next year. We need to keep most of them around to support older versions of Firefox (particularly ESR 52, which will continue to be supported for ~7 months).