[derefr]

IIRC tab bars are optimized for the most-common usage scenario where you only have two or three tabs open. In that case, running horizontally across the top/bottom of the screen means each tab gets to be wide-enough to show a large amount of text, maximizing the amount of context the tab can give for what it's about.

Vertical tabs are stuck in a side-bar, and that sidebar has to fight with the main content for screen real-estate, with the tab bar usually losing (i.e. getting shrunk by the user in order to increase the size of the main content.) That means that, even with only a few tabs open, a tabs sidebar can't show very much description text for each tab.

When you have a lot of tabs, a tab sidebar shows more per-tab context than a tab top/bottom bar does. But having a lot of tabs is comparatively rare.

[GoOnThenDoTell]

I’ve never met anyone who only had a few tabs open at a time

[derefr]

It’s less “having only a few tabs open at once” and more “having a few tabs per window, in many windows.” People who exclusively use tabs are rare compared to people who mostly use windows and sometimes use tabs.

Remember, the default behaviour in all major browsers is to open external links from other applications in a new window. So, if you’re the regular “go with the flow” kind of computer user who presumes the defaults are defaults for a reason — and are a bit lazy in cleaning up your windows, and you use at least one external app (e.g. a mail client, a piece of collaboration software, etc.) — then even if you yourself prefer to open tabs, you’ll end up opening new windows quite frequently as well.

And, if you don’t care where you open each new tab, you’ll just end up opening it against whichever window was most recently opened; rather than having a dedicated “tabs I opened” window.

(As a person who bothered to install the “Merge Windows” Chrome extension to replicate the feature in Safari, I genuinely don’t get these people — but they really do exist, and are even seemingly in the majority.)

[CRConrad]

Not only Firefox but AFAIK most browsers -- at least on Windows -- have interpreted "target=new" as new tab, not window, for several years now. "Open in new tab" is also the first alternative on the right-click menu, before "Open in new window".

Also, BTW, Firefox has had a Tabs menu for years now; both vertical and hides itself away automatically. (Though perhaps you still can't save any vertical space with it: AFAICR it lives on the tab bar, so if you hide that you lose the menu too.)