The deal is, quite simply, that the terms under which Amizilla must be developed in order to get the bounty---OS3.x compatibility most notably---kill the project as surely as 1 plus 1 makes 2. SkyOS apparently has a better infrastructure so it can carry the weight of a complex program like Mozilla far more easily. Enough has been said on this matter elsewhere (you can look up the discussions on this site), so I am not going to rehearse the arguments presented there.
