Why does my live site have formatting errors?

I have uploaded my site, made changes, re-uploaded, and then did that several times. Now, all of a sudden, the appearance of my fonts when I publish the amended site looks different than it does on iWeb - and not in a good way. How can I fix these formating problems? I am uploading to Firefox. In Safari the formating looks ok. I will have end users on Internet Explorer as well.