It might be a result of the fact that Firefox is a beta, or it may be an HTML issue on the site. Not sure, but I thought I'd give you guys a heads up. It's very minor.
Well the main page for Macrumors is showing up on my Firefox 3 in Windows XP Home SP2 with the center column spanning the entire page and under the side columns. This does not show up in internet explorer and I am not at my Mac so I can not test in OS X.
Now it has gone back to normal. But reloading the page did not work rather I had to open Macrumors in a new tab.