Why is there a space on top of unordered list items?

[tachra]

PROBLEM: There is a space on top of unordered list items. This space appears in Firefox 18.0.1HTML: \[code\]<ul><li> <a href=""> <h3>Lorem ipsum</h3> <p>It is a long established fact that a reader will be distracted by the readable content</p> </a></li><li> <a href=""> <h3>Lorem ipsum</h3> <p>It is a long established fact that a reader will be distracted by the readable content</p> </a></li><li> <a href=""> <h3>Lorem ipsum</h3> <p>It is a long established fact that a reader will be distracted by the readable content</p> </a></li>\[/code\]CSS:\[code\]ul { list-style: square; margin-left: 20px}\[/code\]WORKING DEMO: http://jsfiddle.net/vpdd7/