Does CCleaner have this problem while Firefox is still running or when all Firefox windows have been closed? What is it that you want to clean from Firefox? For example: browsing history, cookies, cache of previously viewed pages/images, etc. Firefox has internal features to clear these either on demand or at shutdown. For more information, see: [[Delete browsing, search and download history on Firefox]] Note: When setting up a clear, please be careful to exclude categories of data that you want to preserve. If in doubt, ask us.