Earlier last week, Mozilla updated the Firefox 3.7 wiki to show potential design changes for Firefox 3.7, and today, updated their wiki to show new mock ups for Firefox 4.0 Windows designs.
The designs are changed to fit better into Windows 7 - as they want to "embrace glass" which is a major goal of Mozilla for the next Firefox release, as well as making them feel more a part of the OS. The designs also work on Windows Vista. A separate design would be in place for lower versions of Windows.
The images below show the ideas that the Mozilla team has, for some potential tab layouts/UI redesigns. Remember that these mock ups are different than the 3.7 ones posted last week. Compare them on the Mozilla Wiki.