because when you have more than 8 tabs open on a horizontal tab row, the tab handles start to become narrower and tab titles become unreadable and almost useless. with vertical tabs tab titles can be as long as you see fit, and the tab title does not take away space from other tab handles so more can fit. essentially its more space efficient I think.

but I don’t use it because my firefox theme breaks down when I set up vertical tabs, and everything will be white, even though I don’t even use userchrome customizations