no errors doesn't mean anything for IE. you're going to have to figure out exactly what differences between IE and firefox are. start by using this for ie http://getfirebug.com/lite.html
The report on that page (as of this writing) shows you've got a combination of parsing errors, CSS intended only for Mozilla browsers and deprecated CSS.
It's good your checking for page errors, but always validate the HTML and the CSS via the W3C validation sites and resolve those issues as well.
This post is a little dated, but I took a look at your website in IE7 on Win XP and Firefox 3.5.5 on OS X 10.6.2.
Honestly.. they look very similar, if not identical.
I looked at your code and saw that you weren't using conditional IE statements » to embed a CSS file with tweaks only to be applied when the site is viewed in IE6, 7, or 8.
Perhaps it was the results of validating your code (if you did; I didn't check).
If there are still a few design issues bugging you out in any version of IE, refer to that link above and learn how to use IE-only conditional tags.
If you want to read a great blog post on how to use the conditional tags (less technical and geeky than the MSDN entry), check out this post: