You both are kinda wrong.
His donation was first revealed in 2012, and most Mozilla veterans learned about it back then. Mitchell Baker probably put it best: it surprised them because they had no inkling that he held these views. He did a good job of not expressing those views professionally, and he has always been supportive of Mozilla's policies regarding equality. So his Prop 8 views didn't directly affect his role at Mozilla, and people at Mozilla were okay with him holding those views because it didn't get in the way.
I was active with the Mozilla project some years ago (as a volunteer code contributor), and I know that there is a wide variety of views there, including some developers who held strongly conservative views and many who held strongly liberal views, and they were open about their views. This stands in stark contrast with Eich, who never revealed what he thought politically. This is likely why most at Mozilla were fine with it--they know from first-hand experience the Eich keeps his private and professional lives separate. The few employees who were calling for his ouster lately were all names that I did not recognize--they were either not involved in engineering and/or were newcomers.
On the other hand, it is also true that now that there is this controversy, Eich can no longer function as CEO. He's become a lightning rod, and as long as he remains in that role, it's going to a distraction. That's the sad reality of it--I don't like that it's come to this, and the Mozilla board didn't like that it's come to this, but the fact is, this controversy has become an all-consuming distraction, and Eich knew this. This is why he voluntarily stepped down, and this is why he rejected the board's plea that he stay on in a different role.
It's tragic, really. And I say this as a liberal who supports gay marriage: The torch and pitchfork mob crowd grossly overstepped. This was an act of retribution against someone who made a mistake, and all that it's done is damage the goodwill that the LGBT movement has earned over the years and give ammunition to conservatives.