>>6
Install Stylish or user CSS equivalent. Then,

@namespace url(http://www.w3.org/1999/xhtml);
@-moz-document domain(secretareaofvipquality.net) {
  *  { font-family: Mona !important; }
  h1, h1 strong a, span, .logo, h2, h2 a, h2 small, 
  small a, .rules ul li, .deletebutton a { font-family: Sans-serif !important; }
  pre, code { font-family: Monospace !important; }
}

I only use THE GREASY MONKEY, but it works well enough for me.

// ==UserScript==
// @name           Kareha IPAMonaPGothic
// @namespace      Kareha       
// ==/UserScript==
function addGlobalStyle(css) {
    var head, style;
    head = document.getElementsByTagName('head')[0];
    if (!head) { return; }
    style = document.createElement('style');
    style.type = 'text/css';
    style.innerHTML = css;
    head.appendChild(style);
}

// One word, the forced usage of Mona. Thread Over.
addGlobalStyle('.replytext { font-family: IPAMonaPGothic !important }');