Racism and sexism are often considered firable offenses. For many, this feels no different. The fact that its tied to a "religious belief" or is still something that can be voted on makes it no less bigoted.

His poor response didn't help: "if you disagree with me you're oppressing the poor Indonesians who don't have a mouthpiece!". What an asshole.

The fact is, donating to support prop 8 is bigotry in and of itself. Many have said no one experienced overt discrimination, but all that really means is that he wasn't bigoted to the victims' faces, he quietly donated to a cause with the intent of stripping people he calls his friends of their rights. He apologized for hurting his LGBT friends, but stops short of actually apologizing and then justifies his beliefs with those of a 3rd world country.

Free speech has nothing to do with this, this is someone whose actions many people, both inside and outside of the organization, found incompatible with what they saw as Mozilla's values, and who can blame them?

Tl;dr opposing gay marriage is widely seen as a despicable and regressive cause that many people do not want to associate with