Adjusted default theme now broken

Hello everyone,

I made an adjusted theme for a friend of mine quite some time ago. One based upon the default theme of WordPress (based upon Rubrick). Now it seems to be broken when I view it in Firefox.
First I thought it was a large image that ruined the lay-out, but then I realized a lot of images where disproportionately large all of the sudden.
Anybody any idea why? Because of the new Firefox version maybe? I run 3.0b5 on Ubuntu, but it has issues on an earlier version of FF as well.I haven’t upgraded WordPress recently so that can’t be it (2.3.3 is used on his site at the moment). I haven’t checked the theme in IE6 or 7 yet. I will a.s.a.p.My friend runs 2.0.14 or lower and there it doesn’t display properly either.
You can find the site here. Hope you can help me and my friend out here. I sure can use the help!

Thanks for any insight you guys can give me.