This has been this way for the last few versions or rather since they changed the UI in version 11 I think. This allows you do double click this array to return to window mode or "drag it down" from maximized which will return it to window mode (Windows).
What OS? I have Firefox for Windows and have no unclickable space between the top of the tabs and top of the screen. I've never experienced that before on any release I can remember.
