It's fine that he has personal beliefs against gay marriage. As an LGBT person, it bothers me, but whatever, I probably bother him as well. It's mutual.

My problem is he has expressed he's willing to act on those views, by donating to a cause against gay marriage. While of course it's reasonable for him to act on his views if he wants to, that means I personally don't want to support him because I don't want to help him act on those views. Now that he's CEO, how do we know that LGBT people at Mozilla are getting fair treatment? Would he fairly consider a LGBT person for a promotion? What if two LGBT people get married and invite a bunch of their coworkers, or have a small celebration at work during break? Would he be OK with that? We don't know, but taking away from the fact that he's already acted on his anti-gay views, then I would lean towards that he wouldn't, and that bothers me enough that I don't want to support Mozilla.

There's also the fact that he designed Javascript, which is a perfectly valid reason to hate him :p