AM-DeadLink detects dead links and duplicates in your Browser Bookmarks.
"AM-DeadLink detects dead links and duplicates in your Browser Bookmarks. If a Bookmark has become unavailable you can delete it from your Browser. AM-DeadLink checks Bookmarks from Internet Explorer, Opera, Mozilla and Netscape."
http://www.aignes.com/deadlink.htm
One extra feature it has, it shows the absolute path to the folder that a bookmark lives in. I have a lot of folders in my bookmarks file, and they are heavily nested. Using this tool not only finds duplicate bookmarks, but tells which folders they are in, and it can probe the links to see if the bookmark still points to a live website.
I thought that Firefox's Bookmarks Manager had a function to Go To a bookmark's folder, but looks like not. Perhaps Mozilla does/did?