mobile-config-firefox/content/transparent_reader_toolbar.css

/* Toolbar is invisible until hovered */

@namespace html url("http://www.w3.org/1999/xhtml");

@-moz-document url-prefix("about:reader"){
  .reader-toolbar, .reader-toolbar .button:not(:hover) {
    background-color: transparent !important;
    border: none !important;
  }
  .reader-toolbar:not(:hover)>*:not([open]){ visibility:hidden; }
  body{ text-align: justify; }  
}