I have a triangle (JSFiddle) using this CSS:
.triangle { width: 0; height: 0; border-top: 0; border-bottom: 30px solid #666699; border-le
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.