Because I rarely dedicate a virtual desktop to Firefox alone, and I enjoy the flexibility of a deeper hierarchy (virtual desktops - windows - tabs). Everyone has their own needs and usage patterns. Are you also opposed to file browser and terminal window tabs?

Unfortunately I cannot really justify my interest in philosophical or design-oriented terms. I had simply gotten used to the tab switching behavior in Opera, and have missed that behavior a great deal since having to switch (for reasons that should be obvious).