Yes, this is basically the old Australis theme for Firefox. I've attached an image of a textual mockup I made of the layout, made with Unicode box drawing characters. Ignore the gaps between lines.
Since I started using vertical tabs, I'm addicted to the extra vertical screen space. Given the vertical nature of reading, it makes sense to sacrifice left and right for greater top and bottom.
Idk why I'm sharing this except to say, I agree that the bookmarks bar doesn't need to be a permanent vertical fixture as it's often configured to be.