How do I make a CSS triangle with smooth edges?

For Firefox you can add -moz-transform: scale(.9999); to make smooth edges. In your case:

.triangle {
    width: 0;
    height: 0;
    border-top: 0;
    border-bottom: 30px solid #666699;
    border-left: 20px solid transparent; 
    border-right: 20px solid transparent;
    -moz-transform: scale(.9999);
}

Will do the trick.