Edited answer: (after comments)
“Workaround”, add a transparent outline attribute:
outline: 1px solid transparent;
Tested on Firefox 10.0.2 Windows 7: http://jsfiddle.net/nKhr8/
Original answer: (background-color dependent)
“Workaround”, add a border attribute with the same color of your background (white this case):
border: 1px solid white;
Tested on Firefox 10.0.2 Windows 7: http://jsfiddle.net/LPEfC/