@mibbit has tweeted and blogged that their www server is back up. So the original link works again.
Also I tried 3 different kinds of browsers. In Firefox it shows the youtube thumbnail, in IE and Chrome, it shows a youtube logo that you can click to play the video. If it is a youtu.be link that is posted to IRC, mibbit won't recognize it so you can still just click it. (or maybe it just isn't working for you, anon)