I agree that nobody has come forward with proof that he has done something wrong at Mozilla. Which is why he's still at Mozilla.
But I don't think "hasn't been caught being an egregious bigot at work" is the only bar a CEO of a major nonprofit needs to clear. And whether or not he stays on as CEO, I think it's entirely reasonable for some Mozilla employees to decide that they're better off elsewhere.
Sure, he holds views that some people should be discriminated against, and contributes to political causes advocating that. Lots of people hold those kind of views, and lots of people hold other objectionable views. My objection is to punishing him at work for that, and that alone.
I'm fine with Mozilla employees deciding that they're better off somewhere else, because they don't like their boss for any reason. That's their decision about who they associate with.
I think it's more of a problem when they decide he should be somewhere else because they don't like his opinions, or his political actions. I think that's discriminatory, and a large problem in a free society.