Vivaldi browser supports it out of the box, and vertical is even the default iirc. It's thank to it I got used to vertical tabs and I cannot go back. While it has the drawback of making tabs hoarding easier, just having some tabs open for documentation, GitHub, test pages, quickly brings me to a point where horizontal tabs' title are irrelevant and I have to guess tabs by their favicon. On top of that, most screens are 16:9 and we're reading almost exclusively vertical websites and horizontal tabs take up vertical screen space.