Would you have been fine with it if he had donated money to a campaign to promote that belief?
I didn't say I was fine with his belief, I said that I wouldn't have wanted him to be fired because of it. Nor would I want him to be fired for donating to promote it.
What about if that donation had had your company name attached? Maybe not so much, because that's linking you and your employer to that belief. What about if his job was to be the public face of the company (i.e. the CEO)? Maybe even less, because now the company (and, indirectly, you) are endorsing those beliefs.
If he donated it in the company's name, that would be different, but to my understanding Eich made his donations in his own name, not in Mozilla's name. The contribution database merely notes that Mozilla was his employer at the time. So, for the equivalent question, if I were still at the same company as the coworker I mentioned, and he were promoted to CEO, I would not protest it. Nor would I protest if, as CEO, he continued to make such donations in his name. The important distinction is whether or not the views are being espoused by the person on his own, or by the company -- and I mean literally, officially in the company's name, not some vague "associated with the company because he works there in an important position" sort of thing.
Incidentally, I've been in the "leadership might do things I dislike" position, too. Back when I was part of the team of a startup whose CEO was a Scientologist, a coworker and I insisted that our contracts include clauses that the company not use "Hubbard Management Technology" (look it up, it's hilarious, sad, and offensive at the same time), and that if the company gave money to or bought anything from WISE (a Scientology business group) that it would have to make matching contributions to the EFF.