That may be your problem. Sometimes programs don't shut down properly (thankfully not often) and killing the surviving processes can do the trick. Right click the process name, then select "Kill process tree". That will kill any and all processes associated with it. You may have to do that on each instance of it. I have seen Firefox get hung when it has crashed on me and you cannot re-open FF again until the original process is shut down. This is another reason for an occasional restart of your PC. It clears out any and all processes, orphaned memory, and any junk that accumulates due to our PC activities.