/* Firefox userContent.css */

@-moz-document url("about:privatebrowsing") {
  /* Hide Firefox logo and wordmark. */
  .logo-and-wordmark {
    display: none !important;
  }

  /* Hide search box. */
  .search-inner-wrapper {
    display: none !important;
  }

  /* Hide private browsing myths link. */
  #private-browsing-myths {
    display: none !important;
  }
  .info p {
    margin-bottom: 0 !important;
  }

  /* Hide Mozilla VPN promotional messages. */
  .promo {
    display: none !important;
  }
  .info {
    margin-top: 0 !important;
  }
}

/* Firefox userContent.css */

@-moz-document url("about:privatebrowsing") {
  /* Minimize Private Browsing tab. */
  html.private #search-banner,
  html.private .showPrivate {
    display: none !important;
  }
  .promo {
    display: none !important;
  }
  html.private body {
    background-image:
      url("chrome://global/skin/icons/indicator-private-browsing.svg"),
      url("chrome://browser/skin/indicator-private-browsing.svg") !important;
    background-position: center;
    background-repeat: no-repeat;
    background-size: 48px;
  }
}

Make PB tab show indicator icon only
firefox: Make PB tab show indicator icon only
