firefox renders html differently on two 'identical' machines?

http://www.vmware.com/products/player/  displays correctly on
one linux machine, but is clearly broken on another machine
which is 'supposed to be' identical.

Both machines run gentoo linux, and both are running the exact
same firefox-1.5 release which I downloaded from mozilla.org.

The difference that I see is this:  the column of menu choices
on the right side of the screen (light blue background with
'VMWare Player Home' at the top) appears in the middle of the
screen instead of on the right edge where it belongs.

I don't know enough about html to figure out what exactly is
broken.

Could some html guru take a look at the html on that page and
take a guess at why that part of the page is not rendering
correctly?  I'm assuming that some library is broken on one
of the machines -- but how to find out which one?

Thanks!