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. Please refer to this feature table for examples. This should give you an impression of where the project is headed. More information on why we're doing the project can be found at the FAQ.