The size of the window-drag click target is orthogonal to whether tabs are above or below the address bar. What you really want is a title bar, not necessarily tabs below the toolbar.

And you can get that. In Firefox 29, open Menu > Customize and toggle the Title Bar button at the bottom left. That will give you a normal-height title bar to drag the window with, while still keeping tabs on top to reflect that switching tabs also changes the state of the URL in the toolbar.