From the company’s standpoint, he didn’t leave because of his political opinions - he left because he was causing a shitstorm that was going to hurt the company.
To get away from the politically-charged SSM debate, how would you feel if, say, Ford hired an ex-Chrysler guy, and it turned out that Ford’s customers were so anti-Chrysler that they started kicking up a ruckus? Would Ford be within their rights to tell the guy, “Sorry, but it turns out our customers don’t like you. You’ve gotta go.”?
The answer is yes. Those of us who believe in the market’s power to regulate companies who step out of the mainstream of moral or social views as an alternative to regulation shouldn’t complain when the market works exactly as we say it will. So if the customers of Mozilla are stongly pro-SSM, the company has every right, and possibly a fiduciary duty, to not hire an anti-SSM CEO.
This is entirely separate from the question of whether or not the people doing the protesting are being intolerant or hysterical. In 2008 support for prop-8 was not controversial, even among Democrats. And there’s still a wide swath of the population that is anti-SSM. So it’s not like the guy is expressing Nazi beliefs. He may be wrong (I think he is), but I know lots of people who are against SSM who are not monsters and who should not be denied employment over the issue.