Joe and Gregg, thanks for replying.
It was only one website, and I was clicking on their own "bookmark this" link. That made the bookmarks go into my sidebar. Experimentally, I went back and made a bookmark in the usual way using my iMac top-line-thing, what's the word? Along the top of the screen> Firefox, File, edit, view, history, bookmarks.....that one. Thence, that website's bookmarks behaved themselves as they should.
I therefore conclude that it was that website's "bookmark this" which caused the problem.
I'm reluctant to post the link because there's no point in anyone else carrying out the same experiment. The other reason is that this site, selling fairly expensive services, did not reply to me in 48 hours.
Not impressed with them despite all the hours I've sat here researching their services, so, they're off my list.