Can't open Firefox because a copy of Firefox is already open I have multiple users on my MacBook (OS X 10.9.5). One of them has parential restrictions, but is allowed to use Firefox. Now, despite that, she is unable to run Firefox: whenever she tries, an error box pops up with the message


  Close Firefox
  A copy of Firefox is already open. Only one copy of Firefox can be open at a time.


(needless to say that Firefox was not open, neither by her nor by any other user, and that none of the other users has that problem).

It appears this occured after a recent update of Firefox (to version 39.0).

What could be the cause of this?
How can I sort this out?
Here is what I did to fix the issue:


In OS X Terminal, find your currently used Firefox profile directory (something like):

$ cd ~/Library/Application\ Support/Firefox/Profiles/
$ ls -l

if you have multiple profiles, list the one with the most recent date

$ cd rAnd0m.default
$ ls -la
 
remove the hidden parentlock file

$ rm -v .parentlock

try to start Firefox again


(If the problem was not related to the parentlock file, you will see:
rm: .parentlock: No such file or directory)

If you prefer to use the Finder, just make hidden dot-files visible first, so you can check for the existence of .parentlock and delete it.