CCleaner wont clean Firefox files because it says its open ............. its not and there are no Firefox processes showing in Task Manager
Also make sure another Firefox process isn't running in Task Manager, it's named: plugin-container.exe
That's about the only other process I can think of unless it's some addon/extension you're using causing it.
It would help if you'd supply more information - this info helps the bug fixers/developers hunt down the issues:
* Windows operating version
* Firefox version
* Perhaps a screenshot of the processes in Task Manager when CCleaner states Firefox is running when it's not