Enable Firefox compatibility to link[rel="preload"]

polyfill-firefox-preload.js

/**
 * Firefox has no support to link[rel="preload"]
 * This is a workaround to async load non-critical CSS
 */
if ( navigator.userAgent.search("Firefox") > -1 ) {
  const link_elements = document.querySelectorAll('link[rel="preload"]');
  if ( link_elements.length ){
    link_elements.forEach( link => {
      if (link.getAttribute('as') == 'style') {
        link.setAttribute( 'rel', 'stylesheet' );
      }
    } );
  }
}