I could not find anything.
But when I normaly update CCleaner I usually just run the new version and install it, because I want to keep the same settings keep the same cookies settings in the options tab.
I went and uninstalled CCleaner and then re-installed it and then ran CCleaner and this time it ran normal. It did not say that FireFox need to close.
Not sure if you uninstall the older version then install the new version, but that may be the thing to do when it says a program needs to be closed when it is already closed