The ThirdPartyFileProvidersEnabled setting in OWA mailbox policies controls if Exchange Online mailboxes can access services like Drop and Dropbox for attachments. Office 365 tenants need to decide if they want to allow this kind of access. There’s both good and bad in the feature, but it’s easily turned off if you feel the need.
The new version of OWA (sometimes called Outlook on the Web, or Outlook Web Access) is now generally available to all Office 365 tenants. Although the new OWA has some nice features, you might want to turn off the user choice (toggle) to move the new UI until you’ve had time to prepare the help desk, documentation, and that sort of thing.