It's a business decision. Imagine, instead, the opinion he expressed outside of work is that Firefox sucks and nobody should use it. Should he be fired then?

It's a business decision. Imagine, instead, the opinion he expressed outside of work is that Firefox sucks and nobody should use it. Should he be fired then? As a CEO, your opinion and perspective is MARKETING. You determine if customers stay or leave. And causing customers to leave is obviously a fireable offense.