If your position-x is 0, there's no other solution than writing :
background-position: 0 100px;
background-position-x is a non-standard implementation coming from IE. Chrome did copy it, but sadly not firefox...
However this solution may not be perfect if you have separate sprites on a big background, with rows and cols meaning different things... (for example different logos on each row, selected/hovered on right, plain on left)
In that case, I'd suggest to separate the big picture in separate images, or write the different combinations in the CSS... Depending on the number of sprites, one or the other could be the best choice.
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