pkill -9 -f firefox won't work, what can be a possible explanation?
My friend was running firefox on linux mint, and it froze and he used xkill to kill firefox. But still it shows up in htopps -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.
Stuck in kernel code, possibly because they tripped an assert. Even if not, if your distribution enabled hung task detection, the kernel will log backtraces for these processes eventually; by default, after 2 minutes of being stuck.
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.