In the Favorites bar look for the ⌵ symbol to the right of name of the Favorites list and click on it to reveal the list of items in that list. At the bottom of the list you will see "Open In New Tabs" — click that to open everything in the list each in its own tab
If you are looking at the Favorites menu in Safari's sidebar, Control+Click or right click on the "reveal" arrow and the option "Open In New Tabs" will appear.
Thanks, JM. Alas, that is what I'm currently doing and indeed that works as divined.
But what I was looking for, I fear, may have been overtaken several OSs ago-- I suspect sometime just as tabs were becoming common. As I recall, when in Safari Sidebar/Edit there was a small square one could click/enable and thenceforth all bookmarks in that folder would open when the folder is clicked. And those folders were identified by a small square to the right of its name. In fact, I still have 3 of them in my Favorites Bar.
I just checked an
old 10.10.5 volume and that feature is not there.
The only difference I detect between that old technique and the current system, is that now I must first click on the folder, scroll down to the bottom, and then click again.
I suppose this means that Apple can make changes without my approval. Cheeky devils...