Help needed

I’m trying to split my site in two different sections: ‘top’and ‘bottom’. I’ve given them both a different backgroundcolor. In IE the background colors are shown correctly. In Firefox however only the header is in the top section while the rest is placed in the bottom section. Anyone knows how to solve this?

The URL for my testblog:

http://neve.roeleert.nl/

The URL for my stylesheet:

http://neve.roeleert.nl/wp-content/themes/BreedisWreed/style.css