This is due to FF and IE having different ideas of how to define defaults in padding/margin. The easiest way to overcome this is to use "conditional statements"...
By using a conditional statement, you're telling the browser to apply certain styles ONLY if it is a certain browser. All other browsers will read out of the normal stylesheet.
Here is an example, you can edit and make adjustments as necessary to suit your needs.
Also, I failed to mention that since the Conditional Statement is in the form of a HTML comment, you cannot place it in the CSS. It must be placed in the HTML of the page (between <head> and </head>).