100% font width related.
Here's a screenshot of your Linux results (bottom) with Firefox/Windows (top):
http://img199.imageshack.us/img199/4036/fontwidthslinux.pngAs you can see -- the font being used by the browser is absurdly wide.
The stylesheet defines:
Trebuchet MS, Helvetica, sans-serif
This has come up before, but it's really hard to "fix" without changing the font, as the Windows and OSX fonts the browser chooses are fairly similar (if not identical) in width.
EDIT: based on recommendations here I changed it to:
Trebuchet MS, Liberation Sans, DejaVu Sans, sans-serif