Firefox is going for at most N+1 cores worth of processes, as I understand it (one per core for websites and one extra for compositing and the UI) and is doing

Firefox is going for at most N+1 cores worth of processes, as I understand it (one per core for websites and one extra for compositing and the UI) and is doing to use cooperative threading (Quantum DOM) to improve responsiveness within those processes without the overhead of running a process per website.