Hi,
First, you should always design using Firefox because it is the most standards compliant, then if find problems in IE rendering you can use the IESpecific.css file for IE 6 or IE7Specific.css for IE 7, and it is also possible to create an IE8Specific.css, put it in your skin folder and then in layout.master add IncludeIE8="true" on the IEStyleIncludes control in layout.master
These IE specific css files can override anything you need to make it look right in IE.
Never design first in IE or it becomes much harder to solve problems.
Hope it helps,
Joe