Webkit does still exist :)

Also, Firefox being free software, we can expect forks to appear the very day Mozilla falls. I even wonder if having a swarm of small implementers wouldn't be better for the standards than having three big ones who discuss among themselves, then decide what the standards should be. Maybe browsers would stop getting more complicated than OSes and go back to essential, that way. I would really love seeing web engines getting simple again so that third party developers can contribute to them - and that's certainly a direction many small implementations would push toward.

Of course, this is only speculation. At least, it's not guaranteed doom.