> merging the contents of folders with the same names

Merging/Removing duplicate bookmark *folders* is IMHO a rather different task than removing bookmarks.
(Actually, I consider already the option to "move" instead of removing the bookmarks a questionable feature.) It would require a completely different UI.
Currently, it is not implemented, and I have neither plans nor time to implement it (if somebody really wants to implement it, please contact me by pm or on github.)

Concerning the bug that the moving does copying instead of moving: I cannot reproduce it.
Technically, it uses this function https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/API/bookmarks/move and if this does not do the claimed thing, I would say it is a firefox bug (which should be reported to firefox), or perhaps you have some strange folder which is e.g. immediately re-filled by some other extension or something similar I cannot reproduce.
But it seems that this part was removed from your comment, so perhaps you could not reproduce either.
If you can, please report details about the bug on github (or send me a pm).