Firefox "not responding", can't force quit, doesn't show in activity monitor
This is what I answered in a similar question. It did the trick for me.


  I had the same problem with NetBeans and this is what worked for me:
  
  sudo killall launchservicesd sudo killall Dock
  
  I hope this helps.
  
  I based my answer in this post: Can&#39;t kill Preview app and in previous knowledge.


Check my answer here: App crashes, dock icon remains, can&#39;t reboot, yet no process in ps aux to kill.