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.
kill takes a process ID (i.e. a number) not a process name. Either find the right PID with ps first or use killall, although be aware that killall does exactly what it says: kills all processes matching the string it is given. If you only want to kill one of several Firefox processes that isn't what you want.