No one is saying he shouldn't have been allowed to donate money to prop 8. Freedom of speech means you are allowed to say whatever you want (i.e. saying something should not be a crime punishable by the government).
That does not mean that everything you say should be void of consequence in the realm of public opinion. I for one am glad that Brendan Eich was allowed to make this donation. Now I know that he stands for a position I view as indefensible, and I can choose to remain far away from him and the company he now leads.
Mozilla supports freedom of speech and the free expression of political beliefs. Thus it is consistent with their corporate ethos to hire/promote someone who has "divergent" political or personal beliefs. That's what bdcravens is saying
Personally I think the company's ethos is consistent with NOT suppressing freedom of speech or political expression.