You can't open the application "" because it is not responding

I killed Firefox on my MacBook Pro laptop running OS X Yosemite (10.10.5) by clicking on the Apple icon at the upper, left-hand corner of the screen then selecting Force Quit and then selecting Firefox and clicking on the Force Click button.

Udemy Generic Category (English)120x600

Force Quit Applications

But when I tried to reopen Firefox, I saw the message 'You can't open the application "" because it is not responding.'

Can't open the application

I clicked on OK and used Force Quit Applications again. I saw Firefox in the list and again forced it to quit. I then clicked on the Firefox icon in the Dock at the bottom of the screen again, but Firefox didn't appear to open. I double-clicked on it again, but it still didn't appear to open, though it appeared in the Force Quit Applications list again with "not responding" appearing next to its entry in the list of open apps. I tried clicking on the Firefox icon in the Dock a third time and this time saw the 'You can't open the application "" because it is not responding' message again, though reopening Force Quit Applications again did not show "not responding" next to the Firefox entry in its list. Nor was any other application listed as being unresponsive. I tried the Force Quit step again. Firefox remained in the list until I closed and reopened Force Quit Applications. Though Firefox wasn't in the Force Quit Applications list, I did see a process associated with Firefox when I used the ps command to view processes running on the system.

$ ps -ef | grep -i Firefox | grep -v grep
723184451  6861     1   0  8:46AM ??         0:00.03 /Applications/
nts/MacOS/ /Library/Internet Plu
g-Ins/JavaAppletPlugin.plugin -greomni /Applications/
omni.ja -appomni /Applications/ -appdi
r /Applications/ 1635 gecko-crash-server-pipe.
1635 org.mozilla.machname.776527788 plugin

I killed that process with the kill command.

$ kill -HUP 6861

But when I again double-clicked on the Firefox entry in the Dock, the program didn't appear to open, though I could see it in the Force Quit Applications list and I could see a process running using the ps command. I killed that process using the kill command.

$ ps -ef | grep -i Firefox | grep -v grep
723184451  7345     1   0 11:07AM ??         0:00.06 /Applications/
$ kill -HUP 7345
$ ps -ef | grep -i Firefox | grep -v grep

But I still couldn't access Firefox after again trying to start it from the Dock. I then tried opening the Activity Monitor app found in Applications/Utilities, but I couldn't see that open application either. As with Firefox, it didn't appear when I cycled through open applications with command-tab (you can move from one open application to another by hitting the commandt-tab keys repeatedly). Though I did see the Firefox icon in the Dock, I didn't see one for Activity Monitor. But clicking on the Firefox icon in the Dock did not open it so I could use the program. Nor did clicking on it and choosing Open.

I tried the killall command, but that didn't help; Firefox still didn't appear afterwards when I tried opening it.

$ killall Firefox
No matching processes belonging to you were found
$ killall firefox

The results were the same when I tried opening Firefox from a Terminal window with the open command.

$ open -a /Applications/Firefox
FSPathMakeRef(/Applications/Firefox) failed with error -43.
$ cd /Applications
$ open -a Firefox

I checked the system's CPU utilization, but it was very low and Firefox was getting zero CPU cycles.

$ top

Processes: 230 total, 2 running, 9 stuck, 219 sleeping, 929 threads        11:54:14
Load Avg: 1.73, 1.57, 1.56  CPU usage: 0.12% user, 0.36% sys, 99.51% idle
SharedLibs: 12M resident, 18M data, 0B linkedit.
MemRegions: 29950 total, 2690M resident, 121M private, 654M shared.
PhysMem: 8902M used (1607M wired), 7477M unused.
VM: 542G vsize, 1065M framework vsize, 0(0) swapins, 320(0) swapouts.
Networks: packets: 757278/660M in, 807747/207M out.
Disks: 1177060/19G read, 576078/31G written.

7448  ocspd        0.0   00:00.00 1     0    18    1232K  0B     0B     7448 1
7447  CVMCompiler  0.0   00:00.06 2     1    31    12M    8192B  0B     7447 1
7445  automountd   0.0   00:00.01 7     0    31    1468K  0B     0B     7445 1
7444  top          1.9   00:01.55 1/1   0    22    2012K  0B     0B     7444 479
7441  warmd_agent  0.0   00:00.02 2     1    41    1872K  0B     0B     7441 1
7439  mdworker     0.0   00:00.04 4     0    49    6764K  0B     0B     7439 1
7436  System Prefe 0.0   00:00.01 2     0    49    1496K  0B     0B     7436 1
7420  firefox      0.0   00:00.05 22    0    84    13M    0B     0B     7420 1
7409  Finder       0.0   00:00.06 2     0    83    5260K  0B     0B     7409 1
7387  mdworker     0.0   00:00.04 3     0    50    1712K  0B     0B     7387 1
7385  CoreServices 0.0   00:00.01 2     0    49    1284K  0B     0B     7385 1
7368  mdworker     0.0   00:00.38 4     0    49    12M    0B     0B     7368 1
7364  QuickLookSat 0.0   00:00.21 2     0    43    2344K  0B     0B     7364 1
7279  quicklookd   0.0   00:00.17 4     0    98    5216K  0B     0B     7279 1
7231  netbiosd     0.0   00:00.94 5     4    42    1616K  0B     0B     7231 1
7137  eapolclient  0.0   00:00.12 3     0    49    3264K  0B     0B     7137 55
6893  GoogleSoftwa 0.0   00:00.02 2     0    47    1564K  0B     0B     6893 1

I hit function key F3, but the window wasn't on another desktop. I tried logging out, but couldn't log out - eventually a window appear informing me that I couldn't log out because system preferences wasn't responding - I had tried opening it during the troubleshooting process. I used Force Quit Applications to kill it. I then tried logging off again, but saw a similar message, though this time referencing Firefox. So I used Force Quit Applications to kill it. I then tried logging off again, but saw a similar message again, though this time there was no reference to a specific app.

Your Mac hasn't logged out because an app failed to
quit. To try to continue logging out, click Try Again.

To quit an unresponsive app, select Force Quit from the Apple
Menu (or press Command-Option-Esc).

Clickng on Try Again simply resulted in the same message appearing. I used the command-option-escape keys to bring up the Force Quit window and killed the Terminal window leaving only the Finder showing in that window. I then tried logging out by using the command-shift-q keys, but I still couldn't log out. I still saw the message "Your Mac hasn't logged out because an app failed to quit." I had brought up the Force Quit Applications window previously and selected Finder, which allowed me to relaunch Finder, but that hadn't help with the problem, so I finally just powered the system off and on using the power button. After the system rebooted the problem was no longer there. I could open Firefox and other applications and the windows were visible whereas before, if there were windows associated with the applications I was opening they seemed to be invisible ones.