Some people such as Erich Schubert have suggested how to fold entries by editing planet’s CSS.

I offer the following (works in recent firefoxes) as a userContent.css hack for how I fold long entries on planet. Notably different to Erich’s choice, I use 30em (I have a small font choice) and I use the auto setting for overflow so that I don’t get scrollybars if they’re not needed.

@-moz-document url-prefix(http://planet.debian.org/) {
  div.content {
    max-height: 30em;
    overflow: auto;
  }
}