We have covered Bookmark Duplicate Detector to remove duplicate firefox bookmarks, CheckPlaces is another cool firefox addon which checks your bookmarks are valid and the pages still exist, also checks for duplicates and for empty folders and can restore missing favicons.
Check Place is also very easy to use and has some customization, like filtering which bookmarks to include/exclude and also, check for empty folders, duplicates or dead pages. It will help you remove all the dead bookmarks from your firefox and also deletes the duplicate entries of the same bookmark in firefox.
Install CheckPlaces For Firefox [ Works with Firefox: 3.0 – 3.6.* ]