Is it ethical for someone to use/share confidential information from their past jobs in/with their current (same or different) job or Employer?

It is never ethical to share a companies confidential information.
Corporate information is confidential for a reason.
Most companies have a clause you sign prior to employment that specifically warrants against copying, distributing, or sharing any information which is sole property of the company for, which you are employed or have been employed.

NO , its not ethical , but you can use some information without sharing it to advance your work ,and without causing any trouble for the old job .....
Finally its depend

Work Ethics is a very touchy topic; if work is security clearance job then never, if it's for pepsi or some corporation your experience is you capital :)