Which is the world's fastest browser? According to Zimbra, Safari runs fastest, though it didn't beat out Firefox by much. Both Safari and Firefox were roughly twice as fast as Microsoft's Internet Explorer 7.
Of course, Zimbra was testing for how these browsers perform with the Zimbra Web application. Your mileage may vary with other applications. In fact, I'd love to see a wide range of tests for different sites and applications. Who's next?
As an aside, I continue to be impressed at how Zimbra treats non-Microsoft platforms as first-class citizens (along with Microsoft). Firefox is the same way. Both allow you to run on the Mac, for example, without losing any functionality that you'd find in Windows/IE.
It's called great code. Weak developers write code that limps on anything but Windows. Great developers write code that ports well to diverse platforms.