If the political cause is to take away freedom from others, you can expect resistance.
That's revisionist history. Free software, and the greater "hacker" community, has historically been libertarian. It's only recently that authoritarian leftism took over.
What if he was donating to support miscegenation? Would that be over the line?
In a free society, we do not rely on force to produce morally correct behavior, except by force of state. We rely on law instead, which conversely requires complete freedom to participate in the democratic apparatus. Anything that messes with that messes with the very heart of a liberal democracy.
Okay, let me explain that one. I know he says that, and I know Mozilla says that, but "I've decided to leave of my own accord" has simply become such a cliche that it can no longer be distinguished from firing.
In any case, this mostly just supports my view that people don't understand liberalism and why we need it. When an atheist and a christian can sit down together and have a peaceful family dinner without talking about politics, despite the fact that at least one of them thinks that it's a matter of utmost moral importance that the other change his mind, this is a miracle of social cohesion that is crucial to keeping society together. And we're losing it.
Clearly it didn't work out well for him, but I'm not sure it rises to the level of stupidity so much as naïvety.
After all, he'd probably seen plenty of his peers publicly supporting other political causes without problems.
He just didn't realise the situation was "Leader of open source nonprofit, right-wing, publicly political, pick any 2"