In revision 1.31.325, even after closing ALL Mozilla Firefox browsers ( Firefox V 1.50.4), when running CCleaner, a pop-up message shows up indicating: " You must close Firefox/Mozilla to allow the Internet cache to be cleaned. Otherwise, this process will be skipped".
This did not happen before I updated to the latest CCLeaner rev.

Keep the good job.
Thanks.

Press Ctrl-Alt-Delete and see if the firefox.exe process is still loaded.

I just tested this and there are no such issues. It only happens if you have Firefox open or the process is still going.

Further testing, I renamed HijackThis.exe to firefox.exe and loaded it. Next I ran CCleaner and did an analysis. It told me Firefox was still running (even though it was actually HijackThis).

Your Firefox is most likely hanging (due to a faulty extension/add-on). Simply kill the firefox process next time.

Yeah and the warning pop-up will stop CCleaner from causing problems with Firefox as a few people stated sometime ago by cleaning it whilst is it running. The warning is a safety precaution built into CCleaner now so that Firefox won't be cleaned while it's running, and therefore won't cause Firefox to have problems loading pages (if I'm remembering correctly).

Edit:
Brain-fart reply, I basically repeated myself twice in both sentences.

Andaravi, Tarun, Thanks.

I checked your statements and you were right.
Firefox did not appear as an appication but as a process.
As long as the process is not "killed", the message from CCleaner appears.
As soon as firefox process is "killed", CCleaner runs smoothly.
I documented these steps and attached as jpeg files.
CQFD
Thanks again.