This post is as much for me as for anyone else. I am struggle to keep on top of these release notes.
Every version of Thunderbird comes with release notes. For V78 there are now a number as there have been 8 major releases and some minor releases in the 78 series since July 2020 when V78 was first released.
Those notes do not provide details on the security issues addressed in each release and they are covered here https://www.mozilla.org/en-US/security/known-vulnerabilities/thunderbird/