It always bothers me about this whole vertical tab concept, that, on a theoretical level, I’m fully on board with what you’re saying. But in practice, I’ll often have Firefox tiled side-by-side with another window, and then it’s painful for that sidebar to take up any space at all.
I am happy, though, that this feature is being integrated for the people that find it useful.
Hear me out, responsive tabs. Wide browser window? Vertical tabs. Narrow browser window? Horizontal tabs.