mobile-config-firefox/chrome/vertical_context_navigation.css

/* Source file https://github.com/MrOtherGuy/firefox-csshacks/tree/master/chrome/vertical_context_navigation.css made available under Mozilla Public License v. 2.0
See the above repository for updates as well as full license text. */

/* If this won't work correctly can try the v2 file */

#contentAreaContextMenu{ --uc-popup-display: grid; --uc-pseudo-elem: "" }
scrollbox[orient="vertical"]{
  display: var(--uc-popup-display,inherit);
  grid-template-columns: 38px 1fr; 
  grid-template-rows: 0
}

#contentAreaContextMenu::after{
  display: -moz-box;
  content: "";
  -moz-box-ordinal-group: 0;
}
scrollbox[orient="vertical"]::before{
  content: var(--uc-pseudo-elem,none);
  grid-area: auto / 2 / auto / 2;
}

#contentAreaContextMenu menu{ --uc-popup-display: } /* intentionally blank */

#context-navigation{ -moz-box-orient: vertical; grid-area: 1 / 1 / auto / 1 }
#context-navigation > menuitem{ min-height: 38px }
#context-navigation > menuitem > .menu-iconic-left{ margin: 0 !important; padding: 0 !important; }
#context-sep-navigation{ display: none }
#context-navigation ~ * { grid-area: auto / 2 / auto / 2; }
#context-navigation[hidden] ~ *{ margin-inline-start: -38px; }