Try this
#main-container{
margin:20px auto;
...
}

or you might want to start from a dead center example
http://www.wpdfd.com/editorial/thebox/deadcentre4.html
that is known to work with most important browsers

Wow, I looked at your site in both IE and Firefox. Brings back some bad memories. Here is what I do. Copy your current css and rename the copy ie_fixes.css. This sheet will be used to fix Internet Exploder. Here's an example of how to do it:
The first, default.css is a pretty standard way to implement a style sheet. All browsers will read this. The second is only read by IE. All other browsers treat it as a comment and ignore it. IE will read both but will use ie_fixes. You'll have to modify the ie_fixes.css until you get things looking right. Hope it helps!