No, they didn't have the right to harass you. But you did not help by "flipping them off". All that did was escalate it. Had you pulled off and stopped without giving any reaction whatsoever, I wonder what would have happened if you had kept your cool? We will never know. I would have been scared to death and I don't blame you WANTING to flip your finger. I used to do that and felt completely justified for the same reasons. But you just can't do that. And the car following you after the flip off is exactly why you can't.

Every road rage situation requires both parties full participation in order for it to occur. All that needs to happen is for one party to refuse to escalate the situation.