I don't know if it will fix your font size problem, but this is what I have in my userContent.css for Firefox:

    @-moz-document url-prefix(https://news.ycombinator.com) {
      body {
        background-color: #d4d4cb;
        margin-top: 0;
      }
    
      #hnmain {
        max-width: 960px;
      }
    
      .title {
        font-size: 1.5rem !important;
      }
    
      body, td, input, textarea,
      .default, .admin, .subtext, .yclinks, .pagetop, .comment, .hnname {
        font-size: 1rem !important;
      }
    
      .comhead {
        font-size: 0.8rem !important;
      }
    }

Using rem units means it respects the font size defined in my Firefox configuration. The rest is mostly about avoiding ultra wide content.