Yes, there are differences, but they're not fatal (you can
The usual method is to design for Firefox and then make the
changes necessary for it to work in IE. Firefox is more
standards-compliant than IE. You'll find a lot of help for this
kind of problem if you search on webmasterworld.com.
You should also run some www.w3.org validation checks on your
site. There are a number of minor errors in the html, and one in
the css. If you use Firefox, you can add the Web Developer and the
HTML Validator extensions so these validations are easy to do.