Not Cleaning Firefox Cache

I have windows 8.1.

Go to use the Cleaner, it says Firefox must be closed first before it can clean it.

Meanwhile Firefox is not even open.

I shutdown the computer, started it up again, never opened firefox at all

and yet it is still coming up with the same message.

If I click yes to close firefox which is not open it does nothing, even when I click force close.

Didn't do this before, any ideas why it's happening now?

Have you confirmed no Firefox process is running by starting the Task Manager and checking the Process list?

Also look for FF add-ons that may be getting started at boot time.

Try it in Safe Mode, that way if there is some FF process getting auto started, it won't in Safe Mode.

Check to see if the Mozilla Maintenance Service is running. I've never had it interfere with cleaning, but it's worth a look