Use the following style in order to be able to use the width attribute in Firefox: 

<style type="text/css">
span {display:-moz-inline-box; /* Gecko proprietary */
display:inline-block; /* supported by Opera and ? */
width:25%;height:1em; /* width and height required */
vertical-align:top; /* Gecko needs this */
} /* IE, Opera & Gecko slight diff */
</style>

From http://www.thescripts.com/forum/thread161350.html