Div not centering in Firefox, but fine in IE

Hi all. I have a div that I’m trying to center. It’s working fine in IE, but not in Firefox. I have it positioned because I’m using z-index and when I take the position attribute away, it centers, but I have to have a position in order to use z-index. I must keep z-index. If anyone could help me out, I’d appreciate it. Here is my CSS:

.nav {position:fixed; width:955px; z-index:1; margin-left:auto; margin-right:auto; text-align:center;}