mobile-config-firefox/chrome/multi-row_bookmarks.css

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

/* Makes bookmarks toolbar span multiple rows */

/* Dummy variable to support versions 94-96, can be removed when 96 lands */
:root{ --lwt-frame: var(--lwt-accent-color) }

#PersonalToolbar{
  --multirow-bmb-n-rows: 3; /* Control how many rows are shown before scrolling */
  --multirow-bmb-row-margin: 2px; /* Control how much spacing is between rows */
  max-height: none !important;
}

#PlacesToolbar > hbox{ 
  display: block;
  width: 100vw;
}

#PlacesToolbarItems{
  display: flex;
  flex-wrap: wrap;
  /* --uc-bm-padding is defined in autohide_bookmarks_toolbar.css */
  max-height: calc(var(--multirow-bmb-n-rows) * (4px + 1em + (2 * (var(--multirow-bmb-row-margin) + var(--uc-bm-padding,var(--bookmark-block-padding)))))) !important;
  overflow-y:auto;
  scrollbar-color: var(--lwt-frame) var(--toolbar-bgcolor) ;
  scrollbar-width: thin;
}

/* Hide the all-bookmarks button */
#PlacesChevron{ display: none }

/* Add some spacing between rows */
#PlacesToolbarItems > .bookmark-item{
  margin: var(--multirow-bmb-row-margin) 3px !important;
  visibility: visible !important; /* Just in case they would be hidden for some reason */
}