The final post in the thread linked below appeared to answer my question, but the solution offered doesn’t work on my Mac.
I think the poster was asking about ‘selecting individual browser tabs’ to bookmark at once.
It’s not clear whether what was mentioned in the reply referred to that or to selecting a group of bookmarks in the Bookmark Manager:
“You can multi select bookmarks but not bookmarks folder. You need to hold down the Ctrl Key and select bookmarks and it works. Just doesn’t work for folders.”