Crazy
Now all I do is press the menu title ‘Bookmarks’, find one I want to copy using right click, and paste it near the folder I want to put it in (will not go into the folder directly). Then I grab it and place it over the folder title, and there it is - a duplicate copy in a diff folder.
Did this in about 4 foldere, exited FF and reloaded, and they were all still there. Could not do this first thing this morning…
But, this maybe important: FF had a folder at the bottom of the bookmarks list called ‘Recent Bookmarks’, and just before the succesful copy/pasre above, I had just emptied it (which made the folder vanish)