[hgoel]

The motivation for vertical tabs is pretty straightforward, screens are mostly wider than they are tall, browsers are often used in fullscreen mode, yet much of the web does not use much of the screen's width. So it's a better use of screen space to put tabs on the side than on top.

[inigyou]

tabs also contain horizontal text

[thisislife2]

True. And when I first saw that Tor browser (i.e. Firefox) offered a vertical tabs mode without the tab title (and only the favicons), I thought it was the stupidest vertical tab UI design possible. But then I decided to try it for a week, and guess what - I appreciated the extra space, and it wasn't as hard identifying the tabs through their favicons as I thought it would be. And if needed you can always drag the vertical tab sidebar to reveal the tab title. (Note though that I am not one of these people who opens 100's of tabs and keeps it open. In fact, I find too many tabs cluttering and distracting and if I need to scroll to find my tabs I start closing the tabs).