With Firefox, if it was just the d you wanted to change, you could use GreaseMonkey

This example script would also change any thread titles that were 'd' but seems to work for me.

// ==UserScript==
  // @name        gnuD
  // @namespace   http://www.gnuswebsite.com
  // @description change d to D
  // @grant       none
  // @include     http://www.mudcat.org/index.cfm
  // @version     1
  
  function dToD() {
      var links = document.getElementsByTagName("a");
      
      for(var i = 0; i < links.length; i++){
          if (links[i].text == 'd')
              links[i].text = 'D';
      }
  }
      
  dToD();