He did keep his mouth shut about it. Nobody at the company claims that he made any mention of his support for Prop 8. He was outed for it. And interviews with several of his openly gay employees revealed nothing but his support and empathy for them. Yes, he could have been more supportive by opposing Prop 8. He is no saint in my book. But neither is he a monster that must be removed. He's a complex person, like pretty much everyone else I've ever worked with, and it's wrong to other him for one unfortunate but not necessarily malicious opinion.
He wasn't outed - his donation was public. Its not like some secret conversation was recorded and then reported out of context. He didn't accidentally donate $1,000 to oppose same sex marriage. Once called out on it, he didn't apologize or drop his support of Prop 8.
He made a decision and he stuck with it. And Mozilla employees also made a decision. His decision was to oppose their right to be married. Their decision was to oppose his privilege to be their CEO.
I think that is wrong too. Anonymous speech is important, and therefore so is anonymous support for speech. That was a lot more clear to LGBT folks before their opinions gained cultural ascendancy, when they benefited from anonymity when expressing their previously heterodox opinions. It helped them win. And there is much more winning to be done for other good causes that are still taboo.
Anonymous speech is super important. Even terrible anonymous speech. But this wasn't anonymous speech. Maybe it was speech he hoped no one would find. But, he made the conscious decision to speak in a public forum by donating money to a cause he supported. Then, when called out on it, he made the conscious decision to publicly re-affirm his support for that opinion.
I won't argue about donation disclosure policies. Maybe donations should be public, maybe they shouldn't - I dunno. But, even if they should be private, they weren't and I don't believe he should get a pass because in an alternate world maybe no one would have known what he did.