Swiping between open tabs would be wonderful. Implementation could be exactly how it's done in Safari.
If you're in private mode then swiping would only be between private tabs. Swiping normal tabs would only navigate between normal tabs.
This is not the same as https://connect.mozilla.org/t5/ideas/swipe-between-tabs-private-tabs-and-synced-tabs/idc-p/473#M245, which asks for swiping between all open and all private tabs.