I'm running roundcubemail-nightly-20070318. I've found that if I display a message that has content wider than the display or preview pane, e.g. a very wide image or quoted text that doesn't wrap to the next line, no horizontal scrollbar displays in Firefox (2.0.0.3) while it does in IE6 & IE7.
I'm wondering whether "overflow: hidden;" is strictly necessary on line 699 in skins/default/mail.css:
695:div.message-part
696:{
697: padding: 8px;
698: padding-top: 10px;
699: overflow: hidden;
700:}
...removing that one line allows the horizontal scrollbar to display (as necessary) in Firefox and I don't see any undesirable side effects in IE.
Oops, I just found ticket #1455626 http://trac.roundcube.net/trac.cgi/ticket/1455626 (http://trac.roundcube.net/trac.cgi/ticket/1455626) and added this info to it.