Fair enough not having the horizontal space for it. It's usually still the opposite that annoys me (small centred content not making use of space) but of course this depends on zoom level etc. too.

I'm not sure what you mean about not freeing the top bar though? The topmost thing I have is the back/forward/refresh/URL/extensions/menu bar. (file/edit/view/etc. hidden unless I press Alt, but that's not where tabs would be I don't think.)

The space where tabs go (when horizontal) isn't removed in vertical tabs mode. So using vertical tabs doesn't save any extra on vertical space, only uses additional horizontal space.

Well what I'm saying is I don't have it, so either Sidebery handles it or I put something in userChrome, but either way it's something I have not had to think about in all the years I've been using vertical tabs, such that I was not aware of the issue.

edit: the latter (or Sidebery does do it but my previous solution didn't and this is now redundant, I suppose), I have:

    #TabsToolbar {
      visibility: collapse;
    }

    #sidebar-header {
      display: none;
    }

(~/.config/mozilla/firefox/profile/chrome/userChrome.css)