Many thanks for your reply.
If "Saved Form Information" does not delete passwords, that would be great. As I am not 100% sure, I haven't checked this item.
Also, the docs at http://docs.piriform.com/ccleaner/ccleaner-rules/applications-tab, state that Passwords are part of "Saved Form Information". This seems to imply that this selection will delete passwords?
Maybe the user interface could make it clearer whether passwords are going to be deleted or not? (This needs to apply to all browsers, in addition to Firefox.)