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.
Sorry, it was a mistake, I fixed the post. Also I tried many other ways to kill that process. Thanks for the BUSIER tip.