Browsers assign default values to elements. Some elements are assigned different values, which results with their being rendered differently. I know that headings are given different default font sizes and margins. What other elements are given different values?
borders, outline, padding, margin, etc for the best of your design i recommend you to use a reset css there are allot of them i personally use eric meyers reset
For firefox: http://www.oppenheim.com.au/2008/07/06/how-to-view-firefoxs-default-internal-css-stylesheet/ Same discussion on the accessify forum: http://www.accessifyforum.com/viewtopic.php?t=5551