TSGL: Firefox is already running

[Mike Ferrell]

This has become a too often nuisance lately.  I can close up FF and later decide to open it again.  Then I get the following message:
"Firefox is already running, but not responding.  To open a new window you must first close the existing FF process or restart your system."

I can't close it because I can't find it.  Control, Alt, Del doesn't show it.  So I have to completely reboot.  Can someone help?
Mike