I can’t for the life of me imagine why they don’t just do something like this very quick mockup I threw together (ignore the roughness, e.g., the tabs that haven’t expanded to the full width of the tab bar and the inexact spacing of various UI elements)—that is, simply invert the address bar and most of the tab bar.
This design is better in every way: It makes far more conceptual sense, with a tab’s URL being a property of the tab, rather than the other way around and with the New Tab, Show All Tabs, and Tab Group buttons appearing adjacent to the tabs to which they correspond. It also provides an all-around lighter and airier look (if not more actual screen real estate).
The one downside would be less grabbable whitespace at the top of the window chrome for dragging the window around; but if the whitespace on the address bar level were made grabbable too, it wouldn’t be too much of an issue. Furthermore, extensions would have far more room to live in the address bar (where the majority would belong, as most extensions are likely to conceptually pertain to the current tab, rather than extending tab- or browser-level functionality).
