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.
V78.8 https://www.thunderbird.net/en-US/thunderbird/78.8.0/releasenotes/
V78.7.1 https://www.thunderbird.net/en-US/thunderbird/78.7.1/releasenotes/
V78.7 https://www.thunderbird.net/en-US/thunderbird/78.7.0/releasenotes/
V78.6.1 https://www.thunderbird.net/en-US/thunderbird/78.6.1/releasenotes/
V78.6 https://www.thunderbird.net/en-US/thunderbird/78.6.0/releasenotes/
V78.5.1 https://www.thunderbird.net/en-US/thunderbird/78.5.1/releasenotes/
V78.5 https://www.thunderbird.net/en-US/thunderbird/78.5.0/releasenotes/
V78.4.3 https://www.thunderbird.net/en-US/thunderbird/78.4.3/releasenotes/
V78.4.2 https://www.thunderbird.net/en-US/thunderbird/78.4.2/releasenotes/
V78.4.1 https://www.thunderbird.net/en-US/thunderbird/78.4.1/releasenotes/
V78.4 https://www.thunderbird.net/en-US/thunderbird/78.4.0/releasenotes/
V78.3.3 https://www.thunderbird.net/en-US/thunderbird/78.3.2/releasenotes/
V78.3.2 https://www.thunderbird.net/en-US/thunderbird/78.3.2/releasenotes/
V78.3.1 https://www.thunderbird.net/en-US/thunderbird/78.3.1/releasenotes/
V78.3 https://www.thunderbird.net/en-US/thunderbird/78.3.0/releasenotes/
V78.2.2 https://www.thunderbird.net/en-US/thunderbird/78.2.2/releasenotes/
V78.2.1 https://www.thunderbird.net/en-US/thunderbird/78.2.1/releasenotes/
V78.2 https://www.thunderbird.net/en-US/thunderbird/78.2.0/releasenotes/
V78.1.1 https://www.thunderbird.net/en-US/thunderbird/78.1.1/releasenotes/
V78.1 https://www.thunderbird.net/en-US/thunderbird/78.1.0/releasenotes/
V78 https://www.thunderbird.net/en-US/thunderbird/78.0/releasenotes/
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/
Happy reading.
