Ideally, a toggle for auto collapse. With my ultrawide monitor, I’m keen for the sidebar to always be visible. But if you want it to collapse, that should be available as well.
I suppose many responsive sites have a maximum content width that is admittedly a lot smaller than many displays these days. I guess this lets you use a couple more pixels of vertical space by trading the wasted horizontal space.
I have a really wide monitor, so I end up with wasted space on the sides of most websites, so vertical tabs lets me maximize my vertical space on the screen.
It's also clearly still in development and doesn't really work well yet, so while fun, probably not something you'll want to use yet. It's not even at the point where reporting bugs makes sense.
I hope they will still able us to use the old one, I personally prefer to use the menu than have icons at the left, it is some of the things I really love from Firefox.