The article has some nice screenshots and some brainstorming ideas on interface changes.
http://mashable.com/2009/07/27/firefox-40/
Here are some highlights from the article discussing positives and negatives of moving where tabs are located.
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 title bar 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.