custom css and firefox hack

I’m trying to use a specific position in fire fox using the browser back within the custom css…

this is what i have… why is not working?

.main-navigation ul
{
list-style: none;
padding: 0;
width:980px;
margin:0 auto;
position:absolute;
right:433px;
bottom:631px;
}
@-moz-document url-prefix() { .main-navigation ul { right:461px;
bottom:502px; } }