mobile-config-firefox/chrome/minimal_toolbarbuttons.css

/* Create a circular placeholder for toolbarbutton and downscale to hide them */

/* Create a placeholder for buttons */
toolbar .toolbarbutton-1:not([open]),
.titlebar-button{
  background-image: radial-gradient(circle at center, var(--lwt-toolbarbutton-icon-fill) 0,var(--lwt-toolbarbutton-icon-fill) 10%,transparent 15% );
}
/* Hide placeholder on hover */
toolbar:hover .toolbarbutton-1,.titlebar-buttonbox:hover > .titlebar-button{ background-image: none }

toolbar .toolbarbutton-1 > *,
.titlebar-button > *{
  transform: scale(0);
  transition: transform 82ms linear !important;
}
toolbar:hover .toolbarbutton-1 > *,
.toolbarbutton-1[open] > *,
.titlebar-buttonbox:hover > .titlebar-button > *{
  transform: scale(1)
}

/* Urlbar icons, this way they show colors is applicable */
.urlbar-icon{ transition: transform 82ms linear !important; }
#urlbar:not(:hover) .urlbar-icon{ transform: scale(0.3) }

/* The menu button has some margin on non-compact density which creates annnoying transitions */
/* Lets remove that as well as border since this is minimal style anyway */
#PanelUI-button{ border-left: 0px !important; margin-left: 0px !important; }