[Osmose]

The curves are only on the current tab according to the mockups, the rest are still square.

Here's a comparison using the latest UX Nightly branch with two windows at the same width that have enough tabs to switch to "scrolling tab mode" (notice the arrows on either side):

Old: http://cl.ly/image/221j3J1j3R2Q New: http://cl.ly/image/1b1j1m2K2k24

The new one shows 12 tabs and the edge of the 13th, the new one shows 13 tabs, the last one being cut off a tiny bit. The new one looks cleaner to me what with the lack of extra noise with the boxes around the tabs.

(Of course the rest of your comment may still be valid, but the curves/wasted space in the tab bar in general doesn't hold up IMO.)