Why? Why does his opinion on that private political matter, over which he has almost no control matter to you? Would you ask him who he voted for? If he thought the war in Iraq was justified? If he believes in abortion rights? If he thinks there is a god?
And you ask this question because it relates to your use of Firefox how? Because that's the only relation you have to Brendan Eich: you use a product that his company produces. That doesn't make you two best buddies and you now get to ask his opinion on gay marriage or whether he likes a big butt on his womenz.
You want the answer yes, someone else wants the answer "no". What's the point. If you believe in gay marriage, ask your congressmen to make it legal nation wide. Law is law, belief and laws don't always get along. As a citizen of a country you accept the law and keep your own belief. That's how people live.
"Because my moral code says so" could be an answer.
Or: "Well, if you say yes, why? Is there some scientific explanation of why an arbitrary human custom should be offered to everybody as opposed to only some, based on the moral code one has?".