For a list of new features, fixes and known issues in the latest versions of Thunderbird:

See Thunderbird Desktop release notes for a full list. Or for a summary of changes and new features, plus detailed descriptions, see New in Thunderbird Desktop.
See Thunderbird for Android release notes for a full list. Or for a summary of changes and new features, plus detailed descriptions, see New in Thunderbird for Android.

Thunderbird release notes - What's new and known issues