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.

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.

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.