To remove duplicate bookmarks, navigate to the Add-ons Manager, open the Bookmark Deduplicator options page, and press Start.
A popup showing all duplicates will be opened. On confirmation, your duplicate bookmarks will be removed and the amount of removed duplicates will be presented.
Note: Bookmark Deduplicator will remove every duplicate occurrence of the same URL. Consequently the first occurrence of each bookmark will stay unaltered.
With a huge amount of bookmarks, the browser may appear frozen during the process. This is not sign of an error, just wait until the cleanup is done.
An older version (1.1) is available for Firefox 9 and below, although it lacks the confirmation window. See "View other versions" above.