And even he were a major campaign leader against gay marriage, it doesn't necessarily mean he is going to bring his politics into the Boardroom. The founder/owner of the Stagecoach bus company in the UK (Brian Souter) campaigned very strongly and publicly against the repeal of Section 28 (an statute that banned the "promotion" of homosexuality in schools), especially in Scotland, using his personal fortune to run an unofficial 'referendum' on the law there. However, Stagecoach has an excellent record as an equal-opportunities employer, with no-one expressing concern that Mr Souter was using his company as a platform for his own view on homosexuality or that its employment practices reflected it in any way.
Ultimately, limited companies are not Leninist organizations, in the sense of being the personal tool of the CEO. The CEO has to answer to the Board and if he did want to change Mozilla employment practice to discriminate against gays, or use corporate money to finance an anti-gay-marriage campaign, other Board members would have to agree to it.