It is possible to display more than one application on screen at arbitrary sizes, so the shape of the window should be what matters, not the shape of the display. Browsing maximized on a 27" widescreen display is an uncomfortable waste of space. Vertical tabs or not.

Especially if you have an ultra wide screen this becomes really noticeable. Vertical real estate is precious (and which way do pages/phones scroll?) - but horizontal is not.