After the latest update two days ago. CCleaner is telling me that Firefox needs to be closed. A careful check of Firefox indicates it is not open and running.
Also, as a side remark. It has been well over a year since another bug was supposed to have been to the Second Line December 2019. to fix another bug.
The software updater has never worked. Not one program has ever appeared in the Programs needed column nor the Programs Up to Date Column.
Because of this one and two other minor issues , I let my subscription lapse.
I'm not seeing that issue with Firefox here on CCleaner v5.86.
Are you using Health Check or Custom Clean? (Shouldn't make any difference but we'll know which you are using).
It sounds as if your Firefox may not be closing properly (that happens occasionally), or that an extension/add-on is running in the background after you have closed Firefox (some are meant to do that).
Did you check in Task Manager to see if anything Firefox related was still running?
However I do notice that you have a Thunderbird Icon on the taskbar showing an item, and that may be what is causing CCleaner to say that Firefox is still running?
ie. Thunderbird may be keeping Firefox files open.
(You also seem to have 2 instances of CCleaner open at once; again that shouldn't make a difference but not sure why you have 2 open?)