Try going to Preferences->Playback, and click on Setup for Adobe DV. There should be a selection for Aspect Ratio Conversion...try switching that and see if that helps.
Premiere has an option for Hardware or Software Aspect Ration Conversion but it doesn't do anything, both settings produce the same problem. Encore and After Effects have a 16:9 and 4:3 letterbox option and that works.
What hardware are you sending the FireWire into?
I checked it using various Firewire devices but they all act the same in this situation. Pioneer DVD recorder, Sony DV camcorders.
The unique thing with Premiere CS6 compared to CS5.5 or earlier versions is that it allows Firewire previews with the Mercury GPU on, it doesn't force you to turn that feature off and use the Mercury Software setting instead which is lower performance if you have the proper video card.
I just checked it using Premiere CS5.5 and it does the same thing. In 1080 HD Sequences, Firewire out has the 16:9 incorrectly letterboxed, or what I call "double" letterboxed.
With both CS5.5 and CS6, the Firewire preview is correctly displayed in a DV 480i 16:9 Widescreen Sequence.
A little annoying but this is where an old Sony PVM 4:3 broacast video monitor works well in combination with a widescree 16:9 video monitor still. Simply turn off the 16:9 mode and it will dsiplay it correctly when editing an HD Sequence. When using a DV widescreen Sequence you would then click on the Sony PVM's 16:9 mode so it would letterbox correctly.
Surprised Adobe didn't fix this in Premiere CS6