The usual way to resolve Firefox problems is simple to do an 'on top' install. (i.e. reinstall Firefox WITHOUT uninstalling it first). It won't do any harm to give it a try.
I have solved the mystery. My problem was a non-problem. The bookmarks appearing in the side bar are in fact the individual pages from subscribed sites in the bookmarks toolbar. I can only but put my befuddlement down to seasonal overindulgance. Thanks to those who responded and a Happy New Year to all