Nope, Firefox was closed.
Warning
Just because Firefox is closed, does NOT mean Firefox is closed !!!
Sometimes I close Firefox, and later on I might open it again - normally successfully.
When unsuccessful I get a message it cannot be opened because it is running.
I launch Windows Task Manager and the Applications TAB shows no evidence of Firefox, it is closed,
Then I look at Processes and there it is;
I select the Firefox process and click "End Process", then it goes away.
Now it REALLY is closed, and now I have no problem re-launching Firefox.
I strongly suspect that even if the Firefox Application Window is closed, and this application is no longer running, if the Firefox Process has not terminated that might prevent CCleaner from trying to purge the cache.
AND ALSO if a programme (e.g. Firefox) has required Windows to give it exclusive access to a file, nothing else can access that file until that programme has notified Windows to release its control, and if that programme suffers an abnormal termination that file is NOT released and will NEVER be released, even if Task Manager shows no trace of the application nor its process. Nothing can access that file UNTIL you reboot.
I have lost count of the number of times I used WORD to edit a document, and something went wrong, and after WORD closed (either an out-right automatic crash collapse, or possibly multiple clicks on a "Close" button), I have not been able to do anything with that document until I reboot the computer.
Alan