I have no idea whether anyone has thought of this already, so please don’t sue me. Folding vertical browser tabs. The idea is to be able to fold and unfold tabs, thereby dividing the horizontal space available. This would enable you to not only browse more than one website at once, it would also make copy-pasting stuff easier. Or looking up stuff in a dictionary whilst reading a page that uses fancy words. And it would allow you to fully use the screen real estate on large displays.
The image is a very very crude (it took me only 10 minutes to make) mockup of the idea, based on Firefox. It has four tabs, with two of them ‘active’ (unfolded) and two ‘inactive’ (folded). Click the small version to see a higher resolution one. It should give you an idea of that I mean. Let me know what you think.