Some blocks don't display in my WordPress website

I inherited a website created with the Thesis theme for WordPress. Which means I have no access to their paid support forums…

Aside from the hours I have spent trying to figure out how to do minor things that took me five minutes on a “regular” theme, the thing that is baffling me at the moment is why my site now looks considerably different in Firefox and IE.

Yes, I know, the two browsers display certain things slightly differently. I am aware of that. However, they did at one time display the site very much alike, and so I have been ignoring IE as I made changes and viewed them in Firefox.

But now there are blocks missing in IE - even though they are there under View Source.

Well, have a look and see what you think:

Firefox is the way it should look. IE is the nightmare vision.

Any ideas what is causing such a drastic difference?