I'm a little surprised of strange behaviour of Firefox empty span has height:
var height,
width,
infoText = "",
$span = $('<span></span>');
$('body').append($span);
$span.text(infoText);
height = $span.height();
width = $span.width();
$('body').append(' height: ' + height);
$('body').append(' width: ' + width);
reproduced only in Firefox (FF version is 27.0)
in Opera, Chrome and IE height = 0
is it FF bug?