Few months ago, I realized my bookmarks has too many duplicates. It looked almost similar to your screenshot. I had to use this addon to remove the duplicates,
https://addons.mozilla.org/en-us/firefox/addon/bookmark-duplicate-cleaner/
It listed the duplicates that I gradually removed.
It'd be nice if it were possible to edit the bookmarks.json, remove duplicates and then import into Firefox. Unfortunately, in my case, the json
was too huge to make any head and tail of it.
Add them all and use a duplicate remover, there are a few:
https://addons.mozilla.org/en-us/firefox/addon/bookmark-deduplicator/
https://addons.mozilla.org/en-us/firefox/addon/bookmark-duplicate-cleaner/
https://addons.mozilla.org/en-us/firefox/addon/bookmark-duplicate-detector/