Not at all, copied verbatim from my userContent.css file, full path $FIREFOXPROFILE/chrome/userContent.css:

      @-moz-document domain(news.ycombinator.com) {
         .comment {
            font-size: 12px;
            line-height: 1.75em;
            padding: 10px 0 10px 0;
            max-width: 64ch;
            display: block;
         }
         .commtext {
            font-size: 12px;
            line-height: 1.75em;
            padding: 10px 0 10px 0;
            max-width: 64ch;
            display: block;
         }
      }