You can try the Bookmarks Organizer add-on. It’s rather new, but therefor actively developed and meets my expectations so far.

I do however recommend backing up your bookmarks file (places.sqlite - see this article) before you use it to delete any bookmarks, since there may of course be bugs leading to unwanted side effects. Not that I have seen them though.

If you do not use Tags on your bookmarks... and if there is a good program for this that works with Windows Favorites (Internet Explorer bookmarks), you could do this:

(1) Export Bookmarks in a format you can import into IE (Export Firefox bookmarks to an HTML file to back up or transfer bookmarks)

(2) Import the file into IE and run the tool to check and deduplicate

(3) Import the IE Favorites back into Firefox (Import Bookmarks and other data from Microsoft Edge) - you probably need to delete your Firefox bookmarks before importing to avoid further duplication...

Thank you guys! I'll try your suggestions... Cheers!