Solved +1000 dups problem pretty easily and without issue. Thanks so much!
Update 2 yrs later: with Martin's quidance, changed those unreadable pink Visited Links to deep red in Fx:
Settings/General/Colors/Manage Colors/Visited Links. I am happy!

THANK YOU MARTIN! GREAT WORK!
I had over 50k bookmarks to deal with.
Bookmark Dupes did 99% of the work for me!

I did not use expert mode, but I felt this add-on was easy to use and handled my problem (duplicate bookmarks) very quickly with no issue.

I have imported my Chrome bookmarks several times over the years and was dealing with several duplicate copies of my Chrome bookmarks with varying levels of additions in Firefox. I finally decided it was time to clean.

Bookmark Dupes allowed me with one click to find all of the dupes in my library. With a second click, I was able to select all but the newest bookmarks, and with a third click I was able to move them to a new folder called dupes. Then a quick check to make sure that everything was good and I deleted ~7000 bookmarks from my library.

I then ran the "empty folders" option. The only minor complaint was that I had to run the "empty folders" about five or six times. I'm assuming that if an empty folder is inside another folder it will not consider the parent folder empty, but it was pretty easy to to "empty folders" then "select all" then "delete" until 0 empty folders were reported so definitely not a major complaint.

Overall great and easy to use, took less than five minutes to install and clean out my bookmarks library.

Works well. If you just imported a large number of bookmarks (17,000 in my case), wait 5–10 minutes before searching. Similarly, after batch deleting bookmarks, give it another 5–10 minutes before closing the tab.

+ easy to use
+ self explanatory
+ does exactly what it is supposed to do
+ no weird stuff

as always: make backup before any changes. (export your bookmarks)

Works great, thanks!

If it gets Hungarian, you will get the five stars! consider it necessary!

Great!!!!

Works as expected. Nothing fancy and could use some polishing, BUT it is the only extension (for this purpose) that doesn't ask for more permissions than it should. It uses "read and modify bookmarks" and nothing else. As such, even though it is not perfect, it is the best choice, IMHO.

Simple UX and works well!