My friend was running firefox on linux mint, and it froze and he used xkill to kill firefox. But still it shows up in htop ps -aux. He tried to kill it multiple times but it didn't work. See the pictures for explanation. We had to kill power to shutdown, even systemd can't stop that process.
Did you literally type kill -9 firefox? Because the kill command normally takes PIDs not process names. killall takes process names, but process names are not always straightforward. Under normal circumstances firefox would exit when X/Wayland goes away though.
Using the sysrq key in the "reverse BUSIER" sequence when your system won't shutdown/reboot is always better than shutting the power on a running system.