Hi mahavo
Would it add more security if I add to the Thunderbird email config an onion address instead of clearnet address as the the server address?
This thread has lots of great info. Should answer a most of your questions.
Does it mean that all traffic will stay within the tor network only?
Yes.
Does the hidden onion service operator can uncover my IP address if I use a tor browser or Thunderbird email client.
There are attacks that can de-anonymize Tor users.
I found some articles which state that hidden service operator may trace IP connections by modifying tor software installed on the hidden service.
Could you please provide a link to the articles