Betterbird is a fine-tuned version of Mozilla Thunderbird, Thunderbird on steroids, if you will. Betterbird is better than Thunderbird in three ways: It contains new features exclusive to Betterbird, it contains bug fixes exclusive to Betterbird and it contains fixes that Thunderbird may ship at a later stage.
Betterbird 140 ESR is based on Thunderbird's current ESR version. Refer to Betterbird's release notes for details.
