BG Centering Different in IE & FF

I’m new to WordPress. I’m currently designing my first theme (by manipulating the ‘default’ theme) and have run into a problem. Internet Explorer 8 and Firefox seem to have a difference in opinion on what top center means when positioning the background image, is there a solution to this? I don’t know the rules here so I will wait to post code if I need to. Any help would be appreciated. Thanks.