You must have some setting invoked that's doing this on two computers. I see the '+' whether I have one tab or more, but not if the tab bar is hidden, of course.
So, when you click the View menu, you see Hide Tab Bar rather than Show Tab Bar?
Not to give up on it already, but if you start using one of the other methods of opening a new tab, you might just get used to it and never miss the old way.