This is primarily for my own reference. To get a monospaced font in gmail, add this to your firefox userContent.css file:

div.msg div.mb { font-family: monospace !important; font-size: 12px !important; }

textarea.tb {
font-family: monospace !important;
font-size: 12px !important;
}

td.ct {
font-family: monospace !important;
font-size: 12px !important;
}