The way to get multiple copies of a bookmark is to use Copy and Paste (or hold down the Ctrl key and drag the bookmark).

To what folder are you tring to copy the bookmark? Is that a folder you created yourself?

Some folders aren't actual folders, but are so called smart folders that show the result of a database query.

@cor-el: Thanks for trying to help. I'll try to be clearer: I am attempting to copy and paste. As I stated in my first post, pasting does not add the copied bookmark to the target folder. There is a split-second flash of the folder trying(?) to paste but it immediately disappears and doesn't complete. As far as to what folder? A created category folder within Bookmarks, which is, of course, supported. I've seen posts where this was occurring to others but there never was a solution offered. Seems an odd thing but I'll live with it.  :-)

Are you possibly using Sync as that can be cause for bookmark issues ?

If you use Sync then best is to disconnect Sync temporarily during troubleshooting.

This can be caused by a problem with the places.sqlite and favicons.sqlite databases in the Firefox profile folder.

• use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page

If errors are reported with "Verify Integrity", close and restart Firefox or reboot and retry.

If "Verify Integrity" cannot repair places.sqlite, rename/remove all places.sqlite and favicons.sqlite files in the Firefox profile folder with Firefox closed. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder.

• keep a backup copy of places.sqlite in case a new places.sqlite database has to be created

See also:

• https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file
• https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Folder/Directory:
  Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

@cor-el: Nope. Nothing to do with sync. Really, my bookmarks work just fine except for this niggle. As I said, I'll live with it - most certainly before I delve as deeply as you're suggesting...just not worth it, but thanks for your effort. In the meantime, I've discovered a workaround for the few times that I need to do this move. I'll detail it if someone needs it but I somehow doubt it. *It could even be one of those dreaded firefox "addon" side-issues and I'm not going there! So...case (somewhat) solved. Thanks again.  :-)