CSS :before and :first-child combined

Try

#navigation_center li:first-child:before {
    content: '';
}

Edit: I wanted to expand on this answer with comments made by FelipeAls. The original question used :first which is not a valid CSS selector. Instead, use :first-child. Also the order of the pseudo-selectors is important. The first child selector must come first.

I tend to think of :before as a kind of modifier to a selector. It does not actually select an element only the space just before the selected element.