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.

Personally not sure which way I prefer, so I've been letting Firefox keep them at the top for now. Thankfully the design is different enough that it's still easy to identify what browser I'm looking at. I'm frequently in Chrome, Firefox and Safari at work these days.