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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.

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

Add the following
@-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.