Well, looking at it with IE7.0 and FF1.5, there are hardly any noticeable differences. The only major difference I noticed (aside from a few padding differences and font differences) was the last box that displayed the User Online information has the text in a different location.
In IE7.0, it is on top (and that other text is in the bottom right), whereas with FF they are both on the bottom. The box's height is defined, and it is too big for the text anyway. It should not have a defined height so it can stretch based on the amount of content it has.