I don't think he "voluntarily quit". He clearly quit under pressure.
I'm not sure about US employment law, but in the UK this could be termed "constructive dismissal" - this is typically when someone resigns because it has become impossible for them to do their job per the job spec owing to factors outside their control within the company. People can and do routinely win large constructive dismissal settlements in tribunals or courts arising from these sorts of circumstances.
I think it's appropriate to consider this Mozilla case in two parts: firstly, is it reasonable for Eich to hold (or have held) this point of view? In my opinion (and, I would hope, the opinion of all enlightened people), the answer to this has to be "No".
But then secondly - and more pertinently - does Eich's tenure of this point of view constitute sufficient grounds, in and of itself, for his position to be untenable (both ethically and legally)? In my opinion, this has far from a clear-cut answer. I instinctively tend to think that no, it does not constitute sufficient grounds for termination. It's important, incidentally, to remember that a key component underpinning the answer to this question is whether Eich has brought the company (not himself, but Mozilla) into significant disrepute through his beliefs and actions in this matter. And I'd imagine that if a court were addressing this question, it would need to evaluate how much a CEO's views and actions on matters that are a) totally unrelated to the operations and business of the company, b) totally within the law, and c) hardly considered "extreme" (unfortunately) within US society can be deemed to be bringing the company into disrepute.
To use an extreme analogy (merely to illustrate the point in the clearest manner possible), suppose that the CEO of a company that sold mainly into the Chicago market declared himself an ardent Green Bay Packers fan (and harboured a strong dislike of the Bears as a consequence). It's not hard to see that this might potentially have an adverse impact upon sales in the Chicago area. But I doubt that any court would ever rule that this constituted bringing the company into disrepute (in the way that, say, the CEO declaring that his company's products were unreliable and badly-made would).
I reiterate that the example I've given above is deliberately extreme - but I believe it illustrates the underlying principle well.
I wouldn't be at all surprised if Eich and his lawyers negotiated some sort of generous severance package with Mozilla such that he got a large lump sum and/or ongoing payments under the contract, plus possibly other pension benefits, and in return he would formally agree to resign and not bring any actions against Mozilla for unfair dismissal of any kind. I'd be very surprised if - under these circumstances - he agreed to simply walk away (unless there are other things we don't know about).
