> And unless you have a browser full of tabs, vertical tab lists usually have massive amounts of purely wasted white space and are generally much less space efficient overall.

The Firefox and Edge implementations have a collapsible panel for the vertical tabs. I agree if they didn't, it would be worse than horizontal tabs.

However, my pet peeve is that it's now impossible to disable tabs altogether, say when using a tiling WM that implements tabbing itself, controllable with the usual shortcuts. Firefox has an extension that always moves tabs to a separate window, but it's janky.