I use Firefox and Thunderbird on my system. I cant't write if they are the problem or not, because I know the message "Firefox/Mozilla needs to be closed to..." ( I use the german language and the message is in english ;-) ) and this message wasn't in this moment. Yesterday, I installed some Microsoft Updates, but I cant' write if this was in the background while CCleaner worked.
After this error message CCleaner went normal to end of clean process. No crash and I tried some new starts or cleaning processes... OK.