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.
Alright, good to know. I had the idea that they were going in the same direction as Chrome.
It's my understanding that Chrome is 1:1, while Firefox is M tabs on N threads, which has been a decent compromise in my experience so far.