Bookmark Duplicate Cleaner is a Firefox extension that allows you to easily find and remove bookmarks that occur in several places.
To install the extension, click the Install Extension button to the left to go to the mozilla-page for the extension, then click the Add to Firefox button on that page.
To use the extension, first go to Tools → Bookmarks Duplicate Cleaner → Find duplicates
This will scan through the bookmarks and search for duplicates. These will be presented in this window:
The left list is the title (or one of the titles, if the same webpage is bookmarked several times with the same title) of the duplicate bookmark. The right list is the locations of the duplicates.
To delete one or more duplicates, choose a bookmark in the left list and then select all the duplicates you want to delete in the right list
Then press Delete
If you are having trouble with the extension, please go to the issue-section of the project page and create a new issue.