DESERVES 10 STARS IN MY OPINION!

I went from 9451 bookmarks to 1412 in 5 minutes. How I did this:
1. clicked on bookmark dupes on mozilla taskbar. (The double star icon).
2. clicked on "Dupes" in the extension (the first {leftmost} of the 3 boxes).
3. Skipping the box marked "Remove marked bookmarks", which believe to be in the wrong place [IMHO]
I clicked on the "Mark all but the last in each group" (Choises are first, last, oldest or newest)
4. went back clicked on the box marked "Remove marked bookmarks"

Result 8,039 duplicates deleted.

Just had to delete the empty folders and I'm your uncle.

Bob