The three board members who resigned sought a CEO from outside Mozilla with experience in the mobile industry who could help expand the organization’s Firefox OS mobile-operating system and balance the skills of co-founders Eich and Baker.
So it's not about the Prop 8 thing at all.
It's still entirely possible (and more likely if you ask me) that the Prop 8 angle and subsequent objections raised by many Mozillans and Firefox developers have changed their minds about this, but these are big-time board member CEOs we're taking so of course they're not going to say the political position on Prop 8 of the newly appointed CEO is what's making them resign. That's still a bad career move in today's world, but saying "we wanted someone from outside the organization with experience in XYZ" is not.