Yes, it's common, but usually theres enough other board members to vote him/her off, if need be.
Yes, that is legal. But what matters is control of the board and how the structure of the foundation is set up. With six voting board members assuming things like who is the CEO of the company is something that they are allowed to vote on (it normally would be) she could technically be out of there tomorrow. But this is politics, it is not a merit based affair and there is a fair chance that two or more of the board members are beholden to Mitchell and that would protect her from any attempt to oust her.
Not only is it legal. It's fairly standard.
In the US, yes.
Common in the US, but viewed as bad corporate governance elsewhere. A separation of the roles has been a recommendation in the UK Corporate Governance Code since 1992, for instance.
Mitchell Baker’s link on that page is now a broken link?
Not just member, Mitchell is chair of both the parent (Mozilla Foundation) and wholly owned subsidiary (Mozilla Corporation) boards. This is unusual, in my experience.