ps. I would not float the div left. I would add a margin-left to it but just enough to equal on both sides. You want the same amount of space on the left as you have on the right. Floating it left gives you much more space on the right than it does on the left. I would trim down that photo in the header from istockphoto. It is way to big. You don’t need all that space on the right side of it. just position it where you want it to be. then put some margin left on the div. I came up with about 40px and it looks pretty good.
You also have a layout problem in IE7. Firefox is not the culprit and negative margins are never a good idea.
I didn’t look at your code in detail, because if Firefox shows you a mess then you need to revise your coding. You also have a big scroll bar at the bottom in all browsers, which means that you have not centered your content correctly.
Tambien tienes algunas errores de ortografia evidentes.