I've got Firefox 11 running in Windows 7, and the 3D DOM viewer doesn't appear for me; it's not an option in the Developer Tools. I know where to look, because I've used it on another computer under OS X. Do I have to change a setting somewhere to enable it?
EDIT: The solution ended up being updated graphics drivers. I'm on a 17" MacBook Pro with an NVIDIA GeForce GT 330M.