Some killjoys have removed the blink tag from Firefox
You can put jwz's blink in your browser profile's userContent.css to make it work on all websites:  @keyframes         blink {
    0% { opacity:1; } 75% { opacity:1; } 76% { opacity:0; } 100% { opacity:0; }}

  blink {
    text-decoration:   inherit;
    animation:         blink 0.75s ease-in infinite alternate;
  }

(Though it will work a little differently than the native , as the CSS makes non-text elements blink as well.)