I have a DIV with height/width of 15px. It has a background image also with height/width of 15px. The problem is the background image is being cut off a few pixels on right and bottom when zoomed out some levels in firefox. Here's a link and code below. Try viewing it in firefox.
<div id="custom-checkbox"></div>
#custom-checkbox
{
background: url('http://s16.postimg.org/5xacziapd/unchecked.png') no-repeat;
width: 15px;
height: 15px;
}