At a glance I can imagine how much more effective it is at displaying a large number of tabs in the same window.
In fact, I'm finding it quite brilliant. The web at large takes advantage of vertical scrolling, so why shouldn't browsers? All of my bookmarks are already arranged in vertical lists.
Even when faced with the input procedure of navigating my cursor to a browser tab, vertically stacked tabs have a clear advantage (for trackpad/thinkpad nipple users like myself):
To move my cursor up and down (again, with a trackpad) all I have to do is bend a finger at the knuckle. When moving the cursor a substantial distance left or right, I torque my whole wrist slightly or reposition it entirely.
Maybe that's splitting hairs... I'm rather starry-eyed at the moment seeing your Firefox interface. Perhaps I'm missing some disadvantages?