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Just load in the multiple stylesheets separately, or just load the ones in that matter. But Paulie I think nailed it on the head. The menu IS centered, but it’s not wide enough to take up 100% of the space. What it looks like you really need is that navBG.png to be the background of either the ul or .menu class.
I don’t get any of the FireFox hanging problems you mention – seems to work just fine on mine.
Try to get it into Codepen and then we can fiddle with it.