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)