Note that only the active tab is curved; the other tabs have nothing but a line between them. Here's a comparison using the latest UX Nightly:

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

The old one is showing 13 tabs, the new one is showing 12 tabs and the edge of the 13th one. However, the new one is way less cluttered and easy to read IMO.