It seems like the dom.ipc.processCount setting sets the total number of content processes to use.

Note that this means that tabs will share content processes, so a crash in one tab may crash a few others. You can reduce this by ramping up the number of processes, though that can lead to bloat.

No idea how stable this is right now. For me, on Dev Edition with regular multiprocess mode enabled, things work fine and I don't get crashes.