My vote: SIGTERM should force quit. I just used "kill" because Firefox was doing its spinlocking thing and taking 5min to get to each window to ask if I really wanted to close it. (I'll file a separate bug if that doesn't exist... quit should ask ONCE, not once for every window.) Anyway, leaving aside the CPU usage problem, "kill" bypasses all that. It should definitely force a quit.
However: when it receives a SIGTERM it should NOT ask me on next start that it quit "unexpectedly". No, I killed you on purpose. Don't make me kill you again.