whenI close firefox, and later when I try to open firefox again, it says it is already open, although it shows to be closed---don't know when it is closed or not, don't like this

When I close firefox, it closes, then later when I try to open firefox again a message opens and says firefox is already running, ---have to open task manager to close firefox before it will open again, browser may have been open for hours and didn't show to be---why does this happen, don't know when firefox closes or not------