Well if you did the design using IE first then you most likely have IE specific css in the main css files which are not working correctly in the more standards compliant Firefox. In my documentation, I recommend always design in Firefox first then correct things in IE using the IE override files.
I will say that Firefox 3 has at least one bug I've seen, but still if you design in IE first you are creating difficulty for yourself that is harder to deal with later. There is no way to add Firefox specific css but there is a way to add IE specific css to override the standard css so this reason alone is sufficient reason to always design in FF first, because you can always tweak IE later but you cannot make tweaks later that only affect Firefox.
Hope it helps,