The problem is that Firefox and WebKit based browsers seem to align text vertically in different ways when contained in an element that has an even height/line-height and th
This is browser rendering issue. Use line-height 1px less than the given height, for example:
.box { background-color: Blue; color: White; font-family: Helvetica,Arial; font-size: 15px; height: 18px; line-height: 17px; width: 60px; }