Very simple in Firefox (tested in Firefox 60.4.0.esr - any later check toolkit.legacyUserProfileCustomizations.stylesheets etc.):

- use userChrome.css to display ALL tabs side by side:

profile/chrome/userChrome.css :

  tabpanels {
    display: -moz-box !important;
  }

  tabpanels > notificationbox {
    -moz-box-flex: 1;
    border-width: 2px !important;
    border:solid #888;
  }

- with extension like last_selected_tab AFAIR, or your own, to have content-secondary, handled - then hide any browser of other type with styles as well (as by default you have only: tabpanels > notificationbox > browser[type=content-primary] - being the active tab). :)