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
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?