When I open a new window, and right click a bookmarks folder to select "Open in New Tabs", a new tab is created for each concerned bookmark. This sounds logical, but in its stringency gives me one extra tab which contains the start page (as that's what I have "New windows/tabs open with" set to).
It would seem more elegant, if and when the window has a single tab only, and that tab contains whatever "New windows/tabs open with" is set to, to then open the first bookmark from the list in that existing tab, effectively re-using it.
Safari does it this way, and I have come to like that behaviour.