When using Oauth2 for example to connect an existing IMAP account at Google Mail, Thunderbird uses the local http(s) tcp-ports as a call-back-URL. If you run a local web-server this call-back will not work. I'd say this is bad behaviour. It it is really needed to use a http-callback, it should check which ports are available. Or at least give some feedback to the user that the ports are needed. So my proposals are in this order: