How to kill any process in Linux?

To kill any process yo need to first find out the process ID (PID)

For example if you want to kill firfox

#ps aux | grep firefox

This will generate:

username   5365  0.0  0.0  3572  628 pts/0    S    11:00   0:00 grep firefox

Now kill process using PID

#kill 5365

#kill -9 5365

where -9 is special Kill signal, which will kill the process.