Thunderbird Release Notes　Version 68.0, first offered to channel users on August 15, 2019
https://www.thunderbird.net/en-US/thunderbird/68.0/releasenotes/

なお8/17 12:42 現在 68.0はリリースされていない (404 Not Found)
https://ftp.mozilla.org/pub/thunderbird/releases/68.0/
https://ftp.mozilla.org/pub/thunderbird/candidates/68.0-candidates/build3/