Right click >> Open link in New Tab.

Sorry, doesn't do what I was asking. My hope was that when I selected the bookmark, the website would open in a new tab. I know, too much to ask since Firefox has been out for how many years? One of the many reasons I don't use this browser - clunky and doesn't respond to user requests.

Open new windows in a new tab instead has never applied to bookmarks.

Opening a bookmark in a new Tab by left-click was added by an extension. A number of different extensions over the years offered that pref to extend Firefox default prefs.

I have been using a middle-click for that for many years, after the first extension that had that feature died.