Contents slide UNDER the navbar in FF, but not in IE

See: http://www.writeonwhidbey.org/

When you shrink the width of your browser in IE, everything scrunches
up until you get to a the width of the largest image...then a horiz
scroll bar appears, and is necessary to view the hidden parts of the
body. Most importantly, the navbar remains unchanged. What happens in
IE is what I want.

In FF, the body starts sliding UNDER the navbar when you've shrunk the
width of the body to a narrow width. The horiz scroll bar doesn't
appear until the left side of the body has slid all the way to the
left, under the navbar.

How can I get my web to act the same way in FF, as it does in IE, on
this issue?