You must close Firefox/Mozilla to allow the Internet Cache to be cleaned.

CCleaner for Windows CCleaner Windows Bug Reporting
1

I open CCleaner and run it and I get an error message that says:

You must close Firefox/Mozilla to allow the Internet Cache to be cleaned.

I have closed ALL of my programs and CCleaner is the only open program.

Why do I get this error message?

Carl

2

Press CTRL+ALT+DEL and see if Firefox.exe is listed as running, or if you're using some other Mozilla browser like Seamonkey make sure that doesn't show up either.

Sometimes if there's a download in progress in Firefox, or if you've opened up any of the other areas such as the preferences or bookmarks it will still remain running even though the main browser window is closed.