This took entirely too long to figure out.  Hopefully this will help someone else fix it more quickly.  If you have a horizontal ASP.NET menu where all of the buttons are centered and looking great in IE, but will remain left-aligned in FireFox no matter what you do, just wrap the menu control in a -Moz-Center div tag like so:

<div style="text-align:-moz-center;">
     <asp:Menu ...>
     ...
     </asp:Menu>
</div>

References:

http://forums.asp.net/t/1189032.aspx