I'm running Firefox 2.0 on Roundcube-371. I believe this has been an issue for awhile.
The problem appears to be with the pre class in the print.css file. Below is a fix that works for me at least.
I replaced the code below in "skins/default/print.css":
div.message-part pre
{
margin: 0;
padding: 0;
font-size: 9pt;
}
...with this code from "skins/default/mail.css":
div.message-part div.pre
{
margin: 0px;
padding: 0px;
white-space: pre;
font-family: monospace;
}