If you go up to where you main buttons are (ie: the refresh, home, stop, etc buttons) and you right click on any one of them -- it should give you the option to "Customize" ...
After you push customize, a window will pop up and show you some nifty buttons that you can add to your main area up there. They have a "new tab" button that you can add (it does exactly what you need it to - add a new tab - and doesn't require any installation). It looks like a folder with a green plus sign on it (there's also a "new window" that looks similar - but it's a little different).
Anyhoo - once you find it - you can just drag and drop it to wherever you want it to be.

I remember looking forever for an addon that did what I want - only to discover that the "new tab" option is built into the Firefox browser, but it's just not a default button (and that you have to add it yourself - grr)