I'm taking this from Mozilla's blog post, which goes to lengths to portray it as voluntary (in addition to saying it outright at the top):

https://blog.mozilla.org/blog/2014/04/05/faq-on-ceo-resignat...

Since he left ten days after being promoted to CEO, and since the donations were known about in advance, I would argue that he should have known he would have been a compromised leader of Mozilla and if he valued his job, could have stayed CTO as he had been for 15 years. (And if he didn't expect his own employees to write public blog posts calling for his dismissal, then that alone makes him an incompetent and out-of-touch CEO. Whether or not his dismissal was voluntary, if that's the case, he was not qualified for the job.)