This is a gmail support issue. Firefox has nothing to do with gmail accounts or issues.

When I send an email using Gmail, Firefox stores a copy in "Sent Mail", another copy in "Sent", and another copy in "All mail" . Why is this and how can I stop this? Or how can I delete two of the duplicated storage locations? Thank you

You sure you are not referring to the Thunderbird email client? as the Firefox web browser can only view emails by way of webmail site for the email service.