Probably [font width related](http://meta.stackoverflow.com/questions/2181/login-page-layout-broken-ubuntu-ff-3).

Here's a screenshot of your Linux results (bottom) with Firefox/Windows (top):

<img src="http://img199.imageshack.us/img199/4036/fontwidthslinux.png">

As 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 indentical) in width.

Can you install fonts with more standard widths?