Brendan Eich was not fired because he made a political donation. He was fired because the particular political donation he made is evidence that he is a bigot, that he has a broken moral code. There is not, and never has been, any justification for denying gays the right to marry except prejudice and bigotry. Every single non-religious argument against gay marriage has been definitively debunked. Therefore, discrimination against opponents of gay marriage is morally justified, just as discrimination against other kinds of bigotry is.
In fact, even if discrimination against opponents of gay marriage were not morally justified on its own merits, it would be at least as morally justified as the discrimination against gays that Brendan Eich once wanted to see enshrined into law.
In fact, even if discrimination against opponents of gay marriage were not morally justified on its own merits, it would be at least as morally justified as the discrimination against gays that Brendan Eich once wanted to see enshrined into law.