He donated $1000 to Proposition 8 in 2008 (in opposition to gay marriage). It was discovered, and now he's going to step down and is sorry for "causing pain". He stood for what was right, but now he apologizes for it.
"We know why people are hurt and angry, and they are right: It's because we haven't stayed true to ourselves," said Mozilla Executive Chairwoman Mitchell Baker in a statement. "We have employees with a wide diversity of views. Our culture of openness extends to encouraging staff and community to share their beliefs and opinions in public. This is meant to distinguish Mozilla from most organizations and hold us to a higher standard. But this time we failed to listen, to engage and to be guided by our community."
...unless you stand up against gay marriage.