Very useful, but could be more useful (to me). Actually quite cumbersome to use.

- Doesn't test for duplicate tags. Misspellings and the like mean I have duplicate tags.
- blacklisted locations for the folders or default bookmark to delete. Many of my duplicated bookmarks are in the root folder; I'm really not sure how they got there (so it would be the default folder to delete the delete the duplicate). I use tags and not folders for searching.
- whitelisted folders which should not count - I have an addon called "time capsule" which uses bookmarks to remind me that pages exist periodically. Its bookmarks are not duplicates per se.
- The preferences are not a button on the main page.
- I assume you've never actually used your program on a normal user's setup or have OCD. After about fifty deletions one-by-one, even after turning off the confirmations, it gets old. I have less than one percent duplicates but it's too slow to actually use your addon.