on Mozilla/Firefox/Profiles/xxxxxx/chrome/userChrome.css:

/* Hide Horizontal TAB Bar */
#TabsToolbar {
 visibility: collapse !important;
}

/*
 * Description: Auto-hide sidebar.
 *
 * Contributor(s): img2tab
 */

:root {
  --sidebar-min-width: 8px;
  --sidebar-visible-width: 200px;
}

#sidebar {
  position: relative !important;
  min-width: var(--sidebar-min-width) !important;
  max-width: var(--sidebar-min-width) !important;
  opacity: 0;
}

#sidebar-box:hover :-moz-any(#sidebar,#sidebar-header) {
  min-width: var(--sidebar-visible-width) !important;
  max-width: var(--sidebar-visible-width) !important;
  margin-right: calc((var(--sidebar-visible-width) - var(--sidebar-min-width)) * -1) !important;
  z-index:1;
  opacity: 1;
}

#sidebar-header,
#sidebar-splitter {
  display: none !important;
}