4 -> 5 stars: Just cleaned close to 5k bookmarks (that I sync on Firefox between 3 devices) with this add-on. Updated a few ones I still need and deleted the rest, all-in-all took me 15'. The check took less than 30', during which I worked on something else.

2020 review:
- this add-on correctly detected quite some dead bookmarks that '404 Bookmarks' add-on had not seen in this location. So very good.
- Dupes it detected were actual *folders*, so deleting them would have brought havoc in my bookmarks library. Not good.