Dark Launch

Gmail Fixed Width Font; Fixed width fonts in Gmail; Monospace

To force messages to use a fixed width font, create or edit your Firefox userContent.css file.
Code
/home/YOURUSER/.mozilla/firefox/YOURPROFILE/chrome/userContent.css

For Windows:
Code
C:\Documents and Settings\YOURUSER\Application Data\Mozilla\Firefox\Profiles\YOURPROFILE\chrome\userContent.css


Add the following
CSS
@-moz-document domain(google.com) {
    .ii, /* message body */
    .Ak /* textarea when composing */
    {
        font-family:monospace !important;
        font-size:100% !important;
    }
}

Restart Firefox and now messages viewed in Gmail will be displayed with a proportional font properly aligned.