y can use
 * something simpler if you need this */
[equalsize="always"] > * {
  flex: 1;
  contain: inline-size;
}

treecol {
  flex: 1 auto;
  width: 0; /* Don't let intrinsic sizes affect our minimum size. */
}

#certmanager {
  /* This prevents horizontal scrollbars due to <tree> and non-XUL layout
   * interactions */
  padding: 0;
}

/* This matches the <tree> height from dialog.css */
richlistbox {
  min-height: 15em;
  contain: size;
}

richlistbox,
richlistitem {
  min-height: 30px;
}
PK