At best, another Pale Moon is what would happen. They’ve been maintaining their own hard fork of Gecko by themselves since 2016. They clearly have people capable of maintaining a browser engine, though perhaps not quite enough of them. If Firefox were to die, perhaps joining up with Goanna would be the smart move.
At the moment, we have Blink (Chrome), Gecko (Firefox), Webkit (Safari), Servo, Ladybird and Goanna (Pale Moon).