While it would indeed be enormous, it wouldn't be infinite - after all, they are writing Servo (See https://github.com/mozilla/servo/wiki/Design https://github.

While it would indeed be enormous, it wouldn't be infinite - after all, they are writing Servo (See https://github.com/mozilla/servo/wiki/Design https://github.com/mozilla/servo/wiki/Design for details) that's hopefully parallelizable. Of course, whatever that ends up being might not be Firefox.