You are looking at the drag space above the active tab. Only the inactive tabs get expanded drag space. You can see this by putting the mouse pointer above an i

You are looking at the drag space above the active tab. Only the inactive tabs get expanded drag space. You can see this by putting the mouse pointer above an inactive tab and slowly moving it down. The inactive tab doesn't highlight until you are 3 px(?) within the borders of the tab, and until that point, dragging will drag the window. It's not huge, but having that larger drag target does make the window easier to drag. The tab height difference is also only 3 px, so given that Firefox gives 1 px less above the active tab, it's 1 px "wasted" vertical space for a design that looks much more visually appealing IMO.