The board has every right to fire the CEO. That's not at issue. The board could reasonably do that even if the CEO has majority voting control --- iff the board is certain the CEO won't immediately reverse the decision and replace the board. If they fire the CEO performatively (or as a hail mary) knowing the CEO will reverse them, they're causing operational chaos with no upside, and that's not something the board can legitimately do.
There's a subtext in some comments about this that the board can legitimately express a position that it's better that the company not exist than exist with Mullenweg at the helm. That's not a legitimate thing for the board to pursue.