good in IE, bad in firefox – div wrapper?

So take a look at https://www.pgmccullough.com.

In Internet Explorer it looks fine. But in Firefox, the main content is wider, as is the sidebar, shoving the sidebar down to the bottom of the page, and making the whole thing wider than the header.

I feel like the problem must be here:

div#wrapper {
background: #FFF;
width: 776px;
border: 2px solid #000;
border-top: 0px;
margin: auto;
text-align: center;
}

Because in IE, the whole thing IS 776px wide, and in Firefox, it ain’t. Any ideas will be profoundly appreciated!!