Can't Force Quit Firefox

Firefox (version 45.8.0) became unresponsive on my MacBook Pro running OS X El Capitan (10.11.6). I killed Firefox by clicking on the Apple icon at the top, left-hand corner of the page and selecting Force Quit and then selecting Firefox in the list of running applications and then clicking on the Force Quit button.

I then restarted Firefox by clicking on its icon in the Dock. The icon bounced a few times and then stopped, so I assumed Firefox was now running. But when I tried accessing it by holding down the command while repeatedely tapping the tab key to cycle through the open applications, it didn't appear. Nor did it show up in the list of running processes shown by the Activity Monitor.

When I tried to kill any Firefox process that might be running from a command-line interface (CLI), i.e., a Terminal window, with the killall command, I was informed that there were none.

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

Yet Firefox was showing up in the list of applications displayed by Force Quit Applications. If I clicked on its icon in the Dock nothing would happen. When I tried to open it from a command line interface, i.e., a Terminal window, I didn't see any error message, but that didn't seem to make any difference, either.

The only reference to Firefox that I was was from a notice informing me that it needed to be closed along with other programs for an upgrade to Microsoft Office 2016.

$ ps -ef | grep "[Ff]irefox" | grep -v grep
    0 34651 34617   0 16May17 ??         0:17.03 /Volumes/C_OFFICE2016_1533_A1/c
ocoaDialog.app/Contents/MacOS/cocoaDialog msgbox --no-newline --icon-file /Volum
es/C_OFFICE2016_1533_A1/HPES.icns --title Office 2016 Install --text Office 2016
 Install info --informative-text An Office 2016 Update has been delivered to you
r system by the HP ACES CAE Client.^J^JThe following applications must be closed
 to continue:^JFirefox/Safari/Chrome^JMicrosoft Excel^JMicrosoft Word^JMicrosoft
 Powerpoint^JMicrosoft Outlook^J^JClose these applications and click OK to conti
nue. Any application listed above that is still opened will be closed automatica
lly.^J^J*WARNING*^JPlease do not disconnect from the network or shut down the co
mputer during the installation --button1 OK
$

But there didn't seem to be any way to quit Firefox, since selecting it in the Force Quit Applications list and clicking on the Force Quit button didn't remove it from the list. If I reopened Force Quit Applications, Firefox was still there. And there was no way I could kill it from the Activity Monitor, since it didn't appear in its list of running processes. Nor did ps -ef show a Firefox process running that I could kill. I didn't want to reboot at the moment, since I had work I needed to complete. I was able to reopen Firefox, though, using open /Applications/Firefox.app --new. which was a technique I was able to use when a similar problem occurred last month - see Firefox in Force Quit Applications, but not in Activity Monitor.

$ open /Applications/Firefox.app --new
$ ps -ef | grep "Firefox" | grep -v grep | grep -v Microsoft
723184451 99222     1   0  7:52PM ??         0:07.98 /Applications/Firefox.app/Contents/MacOS/firefox
$