Dropdown not working in FF

I have this CSS:

.master {

}

.slave {
	display:none;
}
.master:hover {
	background-color:white;
	padding:5px;
	-moz-border-radius:5px;
	-webkit-border-radius:5px;
	border-left:2px solid gray;
	border-right:2px solid gray;
	border-top:2px solid gray;
	z-index:2;
}

.master:hover .slave {
	display:block;
	position:absolute;
	-moz-border-radius:5px;
	-webkit-border-radius:5px;
	border:2px solid gray;
	background-color:white;
	z-index:1;
}

Code (markup):

And this HTML

<span class='master'>Hover me<span class='slave'>to show some text</span></span>

HTML:

Why does this not work in firefox only? When I hover over master in FF or IE nothing happens at all. It works in Google Chrome though.

 

Woops, looks like it was an issue with the doctype. Never mind. I changed the doctype and it works now.