Vertical alignment of text in container: WebKit vs Firefox

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;
}

This is due to differences in how browsers handle subpixel text positioning. If your line-height is 20 pixels but font-size is 15 pixels, then the text needs to be positioned 2.5 pixels from the top of the line box. Gecko actually does that (and then antialiases or snaps to the pixel grid as needed on painting). WebKit just rounds positions to integer pixels during layout. In some cases, the two approaches give answers that differ by a pixel. Unless you're comparing them side-by-side, how can you even tell there's a difference?

In any case, making sure that your half-leading is an integer (i.e. that line-height minus font-size is even) will make the rendering more consistent if you really need that.