Having had some (albeit very few) actual interactions with both Brendan Eich and a few members of the board, I do not think that the board was considering firing him, and I think that Brendan decided to resign entirely of his own volition.
Do you really think that if he would have been fired by Mozilla and then Mozilla had claimed at every corner that he resigned voluntarily, that he would have just accepted that, instead of going to the next media outlet and selling the true story there?