Does Gmail have a setting somewhere to make message text appear in a fixed width font? 'Cause if it does, I sure can't find it...
To force it to use a fixed width font anyway, I added the following to userContent.css in my Firefox profile:
div.msg div.mb {
font-family: monospace !important;
font-size: 12px !important;
}
(Firefox's 'View Selection Source' came in real handy trying to figure out these classnames; regular View Source is a pain on Gmail because the whole thing's Javascript-generated.)