Firefox's tabs will soon sport a sleeker, rounded design.
The new design will likely arrive in the Nightly Channel in the next few days, but if you'd like to test it today, you can download the Firefox UX branch. Retina MacBook Pro users should note that, thus far, the new curvy tabs don't support high-DPI screens.
The new curved tabs look like slightly over-sized, more rounded versions of the tabs Google Chrome has always used. Unlike Chrome, tabs in the background are nearly invisible.
The big question is why? Mozilla's answer seems to be little more than "because we can". On the plus side, the re-skinned tabs will bring a bit of a speed improvement thanks to new graphic elements and faster "paint" times.
For more details on the speed improvements see Firefox developer Mike Conley's write up on the new curvy tabs.
