As posted on wiki Mozilla, Firefox has developing Firefox 4.0 . But this is not final version just for brainstorming and exploration. Firefox release two screenshoot initial Windows theme mockups for Firefox 4.0.

Version 1 : Tabs menu on Bottom
This is similar to current version Firefox 3.5. Possibly add a Bookmarks widget as an upfront replacement for the Bookmarks menu/Bookmarks toolbar (option to turn those on would remain)

Version 2 : Tabs menu on Top
The more contentious Tabs-on-Top concept.

Even this version has it’s positive side, also negative side.
Positives

• 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

Negatives

• 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

What do you think? Beside its functionality, I always take interest in apppearance. Good news for Firefox addict like me