I'm a newb at this stuff, and there are a lot of very talented folks here that can help better, but I will give you a couple of things you can look at:
Is the problem in Firefox or IE, or both? (IE has issues with all kinds of html) If it is just IE, you need to check FAQ's and such to see if you can find the particular issue and code an exception for it.
Make sure that the content is not 'overflowing' its Div and 'pushing' content further down the page. For example if the Div is 180 pixels wide, the content and its padding and margins have to fit within that total. Watch for text areas especially, certain text if not formatted correctly, will 'overflow' and mess things up.
Also, make sure that you have properly 'closed' all your Divs and containers. (each <div> has to have a </div>, etc.) You can inadvertently put content into another container and then it does things you don't expect.
Hope this helps