IE is just forgiving when it comes to rendering. The article suggests that Firefox just plain ignores the CSS div tag altogether and just fills the screen to the edges unless with some brute force coding you can get Firefox to behave.
There, fixed it for ya.
I would classify Firefox as forgiving, rather than IE. FF's obviously coded to display W3C standards, but pages that don't quite do that display fine--most other alternative browsers don't. Load up gamespot.com in Opera: the thing is a mess. Now load it in Firefox: it looks just the same as in IE.