Death threats are terrible, frankly, but I've received death threats on the Internet for saying things about a video game. I hope he reports these incidents to the police because they are serious and the people involved should learn that threatening people with death is not something we allow in a civilized society. Perhaps I should have done the same. It has become far too acceptable to threaten people with death.
Also, your link does not say that Mr. Eich received death threats, though other Mozilla employees do report receiving such threats.