With the latest Mint 9.3 install Thunderbird reports version 68.7.0 (64-bit). My previous Mint 18.3 reported its version was 60.9.0. Current release notes say Version 68.7.0, first offered to channel users on April 8, 2020. Nowhere does it imply beta or testing.
Manjaro however reports Thunderbird Daily 68.7.0 (64-bit) So it appears to be at the same version, however it still says "Welcome to Daily" and also "The Daily release of Thunderbird can be unstable " This is the same daily message I mentioned on Jan. 12th.