Take a look at the following HTML and CSS.
.box {
border-radius: 15px;
border: #333 solid 3px;
background: #333;
}
<div class="box">Hello world</div>
It produces this in Firefox:

As you can see, the border and the background of the div leaves a tiny gap which is visible. I need the border because of a hover state with a different background-color.
How can I overcome this?
This is most likely a bug in Firefox. You could do a simple trick to solve this problem: (it’s not the best solution, I know, but the problem seems to be serious)
markup: a fake border through a ‘wrapper’ div
css: padding does the trick
http://jsfiddle.net/steweb/peYRf/
OR a more elegant way to solve the problems (without add another div) could be adding a shadow on the box of the same background-color to ‘fill’ that white horrible stuff i.e.
http://jsfiddle.net/steweb/Sy2rr/