Screen resolutions are so much wider than the average page, so usually most of that width is wasted; vertical tabs just make much better sense; you don't lose space for the tab names when you have loads open, and you can have so many more before you need to start scrolling.

I know there's been a bunch of posts for this in the past and this isn't likely to be considered any more than the rest, but I felt compelled to make the request, as everything else about this browser is so perfect for my normal day to day browsing.