Not exactly a script, but I have a UserStyle applied to Hacker News (using Stylus: <https://addons.mozilla.org/en-US/firefox/addon/styl-us/>). Here are the best bits:

1. Preserve single newlines that people typed in: Often people hit Return only once, and their intended formatting becomes a wall of text. Hacker News preserves the newline in the HTML.

  .commtext {
    white-space: pre-wrap;
  }
  .commtext .reply {
    white-space: normal; /* fix extraneous whitespace caused by first rule */
  }

2. Vertical lines to visually represent thread depth!

  .ind {
    background-image: repeating-linear-gradient(to right, transparent 0px 39px, rgba(204, 204, 204, 1.0) 39px 40px);
  }