Wrapper DIV collapsing in Mozilla-based browsers

I have a typical Wrapper div

<div id="wrapper"></div>

that I’m using as a containing block on a page. All of the content is inside this div.

It works fine in IE, but collapses in Firefox and Safari. I’ve added a really ugly border and background to make it stand out and it only appears at the top of the page. Here’s the CSS:

#wrapper {    
    width: 800px;
    /* height: 100%; */
    margin: 0 auto;
    background-color: #fff;
    border: 12px solid green;

Has anyone seen anything like this or know how to fix it? I’ve seen this a few times and it’s driving me bananas.