Nope. Half of Mozilla's Board of Directors resigned specifically because he was named CEO, citing "their demands for someone from outside Mozilla with a background in the
mobile industry, presumably to help push Firefox OS and to seize a larger share of the mobile browser market." Gay marriage wasn't mentioned at all. Even if his views were public knowledge at that point, which they weren't, do you honestly believe that half the board would have resigned just because of one, small donation he made to Prop 8 if there was profit to be made? When has a board ever behaved like that? In fact, the board went so far as to defend him against the LGBT activists.
The point is, his entire appointment was characterized by massive internal strife and politics centered in debates about his ability to lead at a critical juncture for the company. This gay marriage thing only got superimposed on the situation later, and now media buffoons are making it seem like it was the reason for it.