> I am baffled how anyone could still be using horizontal tabs
I don't tend to work with a browser full-screen on my larger monitor (2560×1440) instead often using half the screen (so 1280px wide) or there abouts. My other screen is 1080×1920 (standard 1080 but portrait not landscape). In those cases I have less room for tabs on the side. It might be less of an issue but most of the side-tab options I tried had a minimum width noticeably wider than the minimum width of a tab in the standard layout.
What might work for me is tabs that can be flipped from horizontal to vertical at a keystroke, or perhaps even if they reacted to window size (with the default choice being easily overridden).
I don't tend to work with a browser full-screen on my larger monitor (2560×1440) instead often using half the screen (so 1280px wide) or there abouts. My other screen is 1080×1920 (standard 1080 but portrait not landscape). In those cases I have less room for tabs on the side. It might be less of an issue but most of the side-tab options I tried had a minimum width noticeably wider than the minimum width of a tab in the standard layout.
What might work for me is tabs that can be flipped from horizontal to vertical at a keystroke, or perhaps even if they reacted to window size (with the default choice being easily overridden).