Edit -> Preferences -> Interface, uncheck "Open Documents as Tabs". Unfortunately if you do this, maximizing a 'floating' window behaves differently than it used to - instead of maximizing the window within the client area, it maximizes it full screen (covering the menu bar and too palettes). I haven't yet decided which of these new "features" is more annoying.
IMHO Adobe dropped the ball on this one. Just because tabbed windows are popular for web browsing doesn't mean they make sense for Photoshop.