Just after releasing Firefox 3.5, Mozilla is planning for the next version- Firefox 4. Mozilla Wiki showed two mockups for Firefox 4: Tabs-on-bottom and Tabs-on-top. The screenshots are shown below:
Tab on Bottom

Tab on Top

Mozilla Wiki stated some positves and negatives of Tab on Top:
[blockquote]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
.[/blockuote]
In our opinion, Tab-On-Top is a great design, which is very well used by google chrome. It is more content oriented too.
Give your feedback to Firefox developers on the Firefox 4 Mockups discussion page.