IE And Firefox CSS Compatibility Problem
There are many differences between firefox and IE when they parsing CSS file.
When I built this site, I often suffered the pain, that when I designed a HTML page with a CSS style, it looks pretty fine in Microsoft's IE but terrible in Firefox. Here is an example, people often use nested "<div>" tag in their HTML file and so do I. If you also want to do such things and hope people have a nice experience in your website using Firefox you should be careful with your CSS style design.