It was originally scheduled to be released last Friday, but got delayed due to last-minute quality assurance check. Nonetheless, the second beta of Firefox 4.0 is now here. It is available for Windows, Mac OS X and Linux, although the features are not uniform throughout (more on that later).
There are also visual change for the Mac OS X version. It joins the Windows version in having the new "Tabs on Top" interface. Sadly the Linux version is yet to get this new user interface.
Firefox 4.0 Beta 2 feels much faster and smoother than the first beta. So, if you are using Beta 1 consider upgrading. Otherwise, it is the same old warning - "This is not a stable release and you may experience problems". Download link below: