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.
Version A - Tabs-on-Bottom
Possibly add a Bookmarks widget as an upfront replacement for the Bookmarks menu/Bookmarks toolbar (option to turn those on would remain).
Version B - Tabs-on-Top
The more contentious Tabs-on-Top concept.
- Save Vertical Space
Efficiency/Remove Visual Complexity - Right now the tabs have to be connected to something. So we are adding an extra visual element for them to connect to.
Shorter Mouse Distance to Page Controls
- Breaks Consistency/Familiarity - Moving things confuses existing users.
Title is MIA - With the space removed from the titlebar you only get the truncated version in the tab.
Longer Mouse Distance to Tabs - Takes longer to mouse to a tab.
Lost Space - Sandwiched in between the application icon and the window widgets you lose some space.
Combo Stop/Refresh/Go Button.
Attached at the end of the location bar.
Turns green when you start typing.
Blends with the location bar when at rest.
Turns blue on hover.
Turns red when a page is loading.
The proposed iconography is mostly colorless. Adding color to these temporary action driven buttons will make it more obvious something is going on.