Thank you for the guidance, Nukecad. So I looked at the Applications tab and, sure enough, I can see a section devoted to Firefox with five boxes, all ticked. However when I run CCleaner and then open the Advanced Report it still shows all five Firefox options as "skipped". I'm wondering whether there is some setting in Firefox itself which prevents CCleaner from taking control. Interestingly I also use Mozilla Thunderbird, which also appears as an Application in CCleaner, and that seems to have been cleaned without any issue.