Hello,
I haven't done that much experimentation with the NAV menu, but I am about to tackle it over the next month or so. - The techniques I use for investigating all of the classes used in the styling of any DNN element are outlined here:
Troubleshooting DotNetNuke SkinsYou should be able to use those techniques to troubleshoot your menu in FF and work out the best method for centering, hope this gives you a starting point, thanks,