http://www.donotlick.com/wp-content/uploads/2014/04/tab-shap...
1. The curves of the 'aerodynamic' tabs sacrifice pixels in the horizontal ramp-off / -on whereas old 'ugly' rectangular tabs can abutt.
2. A vertical void above the tabs, too shallow into which to put icons.
I'll have no choice but to become accustomed to it but I fear my little 12" screen will become even less efficient.
(2) is the same as Chrome and older Firefox versions - to ensure that there is still some draggable area in the window when it is filled with tabs, and it disappears when the window is maximised.
Please note that I normally use tree style tabs (https://addons.mozilla.org/en-US/firefox/addon/tree-style-ta...) to put the tabs in a sidebar. It may have messed up the layout a bit. (also, the non-standard colour comes from ColourfulTabs)
To me it's just more visually intuitive to have the tab right above the content window, not over all the toolbars. I really can't fathom why the reverse has become the default.