[fdgdd]

There's no need for extensions or userscript for this with Firefox. In ~/.mozilla/firefox/<profile>/chrome/userContent.css:

  @-moz-document domain("news.ycombinator.com") {
      .hnuser { font-size: 20px }
  }

You'll have to restart Firefox for it to come into effect.

[moron4hire]

Yeah, but then I'd have to run Firefox, which is a dying product and probably won't be around in 5 years.