Suffragettes were extremists. Worker-rights supporters were extremists. Gandhi was an extremist. M. L. King Jr. was an extremist. Sans-culottes were extremists.
In the history of civilisation, there are winners and there are losers, and when the revelation of who is who eventually comes, it can be a bit ugly to watch.
Personally, I would have preferred he said his mind had changed over the intervening years and that he had made a mistake. Then stayed CEO, of course.
I think the real reason he left is the fact he had enough ethical integrity to realize his brand damaged Mozilla's brand. That, and his beliefs were unchanged so he could not offer a genuine retraction.
Also, your link does not say that Mr. Eich received death threats, though other Mozilla employees do report receiving such threats.
I don't know if that's a good move by Mozilla as well, although my guess is that it's a personal move from Eich who couldn't handle the pressure and the death threats (which, to be honest, I understand - even if some will say a CEO is supposed to be stronger than that).