mobile-config-firefox/chrome/compact_proton.css

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

/* Small changes to make proton roughly as compact as the old compact mode */

:root{
  --toolbarbutton-inner-padding: 6px !important;
  --proton-tab-block-margin: 2px !important;
  --tabs-shadow-size: 0px !important;
  --arrowpanel-menuitem-padding: 4px !important;
  --panel-font-size: inherit !important;
  --arrowpanel-padding: 0.8em !important;
  --inline-tab-padding: 8px !important; /* 8px is default value since it looks good - lower values compactify tabs horizontally */
}
.subview-subheader{ height: 24px }
menupopup > menuitem,
menupopup > menu{ padding-block: 0.3em !important; }

#urlbar-container{ padding-block: 2px !important; }
#urlbar-background{ margin-block: 1px }
/* This is kinda weird, but it makes the horizontal line between tabs and nav-bar render "inside" nav-bar thus tabs are more visibily separated even if there is less space there */
#nav-bar{ 
  box-shadow: inset 0 var(--tabs-shadow-size) 0 var(--tabs-border-color) !important;
}

.tab-close-button{
  margin-inline-start: 2px !important;
  width: 16px !important;
  height: 16px !important;
  padding: 1px !important;
}

#tabbrowser-tabs{ --uc-tabs-scrollbutton-border: 2px }
#scrollbutton-up,
#scrollbutton-down{ border-block-width: var(--uc-tabs-scrollbutton-border,0px) !important; }

/* OPTIONAL - show audio label in compact mode */
/*
.tab-secondary-label:is([soundplaying], [muted], [activemedia-blocked], [pictureinpicture]){ display: -moz-box !important; }
*/