I understand that Firefox has to be closed to run CCleaner. Normally this is not a problem.
Today (October 09, 2019) Firefox unexpectedly closed while I was checking my Yahoo mail. I re-opened it and I finished, then closed Firefox. When I went to remove some cookies with CCleaner, I was informed that Firefox was still running, when it had been Closed and there was NO indication it was still running with (Windows 10) task manager. I opened Firefox (currently 69.0.2) then closed it twice before trying to remove some cookies with CCleaner. When I went to delete those cookies, I was informed that Firefox was still running. CCleaner opened a window that offered to terminate Firefox. I clicked the option to close Firefox. A few seconds later, I was told that Firefox was not closing and was offered an option to force it closed. I clicked that option and only then was I able to delete those cookies I wanted gone.
What does CCleaner use to determine that Firefox is NOT running? Does Firefox closing unexpectedly freeze this, whatever, to indicate that it did not close? Whose problem this is I don't know, but I have also informed Mozilla that there is a conflict with CCleaner when Firefox closes unexpectedly.
Problem solved, until the next time Firefox closes unexpectedly. I use Firefox almost exclusively.