Não use transparentes em estilos CSS para formas triangulares:
.wrong-triangle {
content: "";
width: 0;
height: 0;
border-top: 5px solid transparent;
border-right: 5px solid transparent;
border-bottom: 5px solid black;
border-left: 5px solid transparent;
}
.right-triangle {
content: "";
width: 0;
height: 0;
border-top: 5px solid rgba(255, 255, 255, 0);
border-right: 5px solid rgba(255, 255, 255, 0);
border-bottom: 5px solid black;
border-left: 5px solid rgba(255, 255, 255, 0);
}
FF gera artefatos (bordas) quando usado transparent
.