There's personal beliefs and then there's giving financial support to crush someone else's. That puts things on a different scale. Abortion in the examples would probably be the closest, but the most 'understandable' part of that is at least it involves termination of life, which is by itself controversial. Prop 8 was about materially preventing families from officially forming. That puts it in a different league of just hatred/bigotry.
A CEO IS the public face of an organization. And that organization, Mozilla, is one whose principal aim is to create an open and transparent substrate for the exchange of ideas and services. That is at odds with his own past actions.
The products and services a company creates is reflective of the people inside of it. His actions have made it difficult for LGBT members to join his organization (and possibly anyone else who is sensitive to such issues). This is ESPECIALLY true for anyone who will routinely be interacting with him. The lack of minorities in tech is already a big problem, and reducing that likelihood at one of the few principal stalwarts of the Internet is a missed opportunity.
