The tabs on top design seems to come from the idea that the UI elements should reflect what they control. Having the back button and other pieces under the tab more accurately reflect that the back button will change that tab, and not the entire browser.
That seems to be splitting hairs. When only one tab at a time has the focus and the controls stay the same.
Now if the tabs had different contents, requiring unique control sets, maybe that makes some sense...
Dammit that sounds like the ribbon UI concept,
"Why did it have to be snakes, I hate snakes"