He actively participated, via monetary contribution, in a movement whose purpose was to prevent a group of people from having the freedom to marry and share in the benefits that society and the government afford the married. This goes against what Mozilla stands for by virtue of the fact that they provide spousal benefits to all couples regardless of sexual orientation.
Brendan chose to leave, because it was hurting Mozilla. This is not a "corporate decision".
It's hard to say whether or not that's true. Such resignations are asked for at least as often as they are volunteered.