Let me explain: I have no doubt that Brendan was an incredible CTO/is an excellent hacker/engineer.
But being CEO is not about that, it's about being a leader. Being a uniter and a communicator. a galvanizer to the corporation/community he was trying to lead.
Forget about his donation a few years ago. In the short time he was CEO, he had the opportunity to weather the storm (so to speak) he and Mozilla Corp found themselves in. But in fact, by avoiding the issue, he dug a even deeper hole.
His one interview about the issue to The Guardian was a complete mess PR wise: http://www.theguardian.com/technology/2014/apr/01/mozilla-ce...
It was a mistake to appoint him as CEO. I'm sad that he left Mozilla, he could have continued to provide a lot of value to Mozilla and the Open Web in several capacities, But he was not "incredibly well-suited" for the job as Mozilla CEO, IMHO.