Chrome wins some tests but loses the most on the start-up benchmarks, which is a real bummer. There was a time where we'd spend literally days figuring out why startup regressed by a handful of milliseconds, and this test shows an end-to-end test taking nearly six seconds.
One idea I saw mentioned is that there may be a bug related to how Chrome reads the system proxy settings (something about an API change between Windows 7 and Windows 8). I think most of the Chrome developers are still on Windows 7; the bots that are tracking startup performance are definitely still Windows 7 as well. So maybe that number is more of a single bug than a systematic thing.
All of the above is not intended to diminish Firefox's impressive results -- just thought I'd provide some background! What these sorts of tests show most is that competition pushes browsers ever faster. If the proxy theory above is right, then maybe this test will inspire Google to invest more into Windows 8 testing, which ultimately benefits users.
(Edit: Firefox won other tests too, I also didn't mean to say this was the only factor.)