Another instance of program is already running

Most of the popular programs make a check before start if it is already running and do not allow another instance of program to run.
Sometimes may happen program /FireFox for instance/ to disappear as window but to stay as running process in computer RAM.
In this case, if try to run Fire Fox, a message tells “Another instance of FireFox is already running. Please close it and try again”
You have to hit ALT+CTR+DEL to open Task Manager.
Then find the program name in the list with processes and rightclick on it.
Choose – “End Process” option and confirm.
Then you may start your program with no problems.