It seems that some Mozilla employees and members of the public deluded themselves into thinking Mozilla was some kind of political charity fighting for social causes, rather than being a technology company.
No one is deluded into thinking Mozilla is a political charity.
They most certainly are a technology company and everyone knows that, so are the companies behind Chrome, Internet Explorer and Safari. As far as I know none of the CEO's behind those companies are openly anti-gay marriage.
The public, and the employees of Mozilla have expressed that this is not an appropriate view for the CEO to hold and will move to other offerings if he is left. He used his freedom of speech to speak out with his wallet against gay marriage. Just as the consumers and employees have expressed that they will use the free market to go to competitors if he is not ousted.
More precisely, a few employees. A similar number of employees are clearly on record as saying they don't agree with Brendan's position but don't think it's incompatible with him being CEO. A vastly larger number of employees didn't say anything in public at all, not least because many of them (e.g. all the non-gay ones) could not do so without being accused of supporting the views in question.
Of course the employees who did speak out in favor of Brendan staying as CEO got death threats for their troubles (see bottom of http://www.twobraids.com/2014/03/the-mozilla-ceo.html for example), because that's apparently how debate goes on the internet nowadays.
It seems that some Mozilla employees and members of the public deluded themselves into thinking Mozilla was some kind of political charity fighting for social causes, rather than being a technology company.