fenix-fox/chrome/autohide_bookmarks_toolbar.css

#PersonalToolbar{
  --uc-bm-height: 20px; /* Might need to adjust if the toolbar has other buttons */
  --uc-bm-padding: 2px; /* Vertical padding to be applied to bookmarks */
}

:root[uidensity="compact"] #PersonalToolbar{ --uc-bm-padding: 1px }
:root[uidensity="touch"] #PersonalToolbar{ --uc-bm-padding: 6px }

#PersonalToolbar:not([customizing]){
  margin-bottom: calc(2px - var(--uc-bm-height) - 2 * var(--uc-bm-padding));
  transform: rotateX(90deg);
  transform-origin: top;
  transition: transform 135ms linear 600ms !important;
  z-index: 1;

}
#PlacesToolbarItems > .bookmark-item{ padding-block: var(--uc-bm-padding) !important; }

/* SELECT BOOKMARKS TOOLBAR BEHAVIOR */
/* Comment out or delete either one of these to disable that behavior */

/* Show when urlbar is focused */
#nav-bar:focus-within + #PersonalToolbar{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}

/* Show when cursor is over the toolbar area */
#navigator-toolbox:hover > #PersonalToolbar{
  transition-delay: 100ms !important;
  transform: rotateX(0);
}

/* Uncomment to enable compatibility for multi-row_bookmarks.css */
/* This would break buttons placed in the toolbar but that is likely not happening if you are using multi-row setup  */
/*
#PersonalToolbar:not([customizing]){
  position: fixed;
  display: block;
  margin-bottom: 0px !important;
}
*/